Rain's Guide to Grouping in
Lord of the Rings Online
Why this Guide?
Dealing with Threat and Aggro
Target Selection, the Main Assist, and Damage
Healing and Incoming Damage
General Logistics, Encounter Preparation, and Etiquette
Advice for Specific Classes
Starting with World of Warcraft, the trend in newer Massive Multiplayer Online Roleplaying games is to make it possible for a player's character to reach very high levels without ever needing a balanced, well-organized group. As they level, players might grab a healer or some extra damage for a "group" quest, but these haphazard teams are not adequate preparation for the level of strategy necessary for really tough encounters.
Unfortunately, by the time players do start facing challenges that require real teamwork, their playstyle is so entrenched in the mechanics of soloing that they have difficulty adapting to the expectations of high-level group play. This guide is meant to introduce you to the basic concept of how effective MMORPG groups generally function, with special attention to some of the unique aspects of Lord of the Rings Online. Since many LotRO players are new to the genre, this guide is as specific and detailed as possible.
The Basic Premise
The basic premise of grouping in an MMORPG is: maximize the effectiveness of the damage output of the group while minimizing the damage taken by group members; and limit the amount of chaos and confusion during the fight so that support characters can fulfill their role with maximum efficiency.
The first concept that any group member needs to understand is "threat." Threat is the general term for a mob's level of desire to attack a particular player. (A mob is technically any "mobile" non-player character in a massive multiplayer game, but typically any creature that might attack a player.)
Many players will use "aggro" as a synonym for threat, although technically "aggro" is the status of being the target of a mob's attacks. In other words, if you raise your threat high enough, you will draw a mob's aggro.
Several actions can raise a mob's threat toward a player and draw its aggro. The most simple is for the player to stay within a certain radius of the mob for a period of time. The greater the difference between a character's level and the mob's, the larger that "aggro radius" is and the shorter the amount of time. In other words, if your level is eight levels below that of the elite troll across the way, merely getting in sight of it may cause it to instantly attack you. If you are eight levels above it, you may have to walk up to him and insult his mother.
In addition to proximity, damaging a mob will increase its threat toward you. The more damage you do to the mob, the more threat the mob will have toward you.
Healing a character that the mob is attacking tends to really irritate the mob. In most games, including Lord of the Rings Online, healing can generate a ton of threat.
Finally, the Guardian class has several attacks which do very little damage but which generate considerable threat. Two Lore-master pets (the raven and the bear) also have attacks which generate threat, but neither pet can tank for very long.
Assume that each mob in a battle has assigned a threat number to each player in your group or your raid. Every time a player causes damage, heals or uses a threat-based attack that number goes up. Each mob will attack the player whose number is the highest for them. Here is an example.
Imagine a two-person group of a hunter and a minstrel. The minstrel does not want any aggro at all, so he hangs back outside of the goblins' aggro radius. The hunter walks up and uses her proximity to get the goblins to attack her (a "body pull"). She then attacks Goblin A, while Goblins A, B, and C attack her.
The hunter's threat number with Goblin A will go up very quickly, because she's beating the snot out of him. Unfortunately, all three goblins are beating the snot out of her. So, when her morale drops fairly low, the minstrel will heal her.
This will cause the minstrel's threat number with all three goblins to go up. In the case of Goblin A, the hunter will be doing so much damage that her threat number will be higher than the minstrel's. Unfortunately for the minstrel, nothing the hunter has done has raised her threat number with them, so, after the heal, the minstrel's threat number with Goblin B and Goblin C will be higher than the hunter's, so B and C will change targets and attack the minstrel.
When that happens, if the hunter does not act quickly (probably by using high-damage attacks) to draw the goblins' aggro back onto herself, the minstrel may very well be defeated.
On the scale of the example above, managing aggro is not terribly difficult. When a full party (or, in LotRO taxonomy, a "fellowship") of six characters are facing combat with four mobs, things become much more complicated.
The simplest solution is to designate a "Main Tank." The role of this character - who should be a Guardian, a Warden, or an appropriately-specialized Champion in most circumstances - is to hold the aggro of all of the actively attacking mobs. The Main Tank watches the health of the other party members. When someone's health starts going down, the Main Tank determines which mob is attacking them and uses a threat-based attack to convince the mob to switch their focus to the guardian. This means that in most situations (notably those where the mobs do not use Area Effect attacks) the Healer can focus most of their attention on the Main Tank. This is the most efficient use of the Healer's power reserve.
If you are in a group with a Main Tank, do not start attacking a mob just because they attack you. Your additional attacks will generate more threat, and make it more difficult for the Main Tank to generate enough threat to pull the mob off of you. Also, do not be surprised if your Healer does not heal you immediately. Their heal might further muddy the mob's threat ratings for the group. More importantly, the Healer knows that a good Main Tank will pull the aggro back off of you before you die. There is no reason, therefore, for the Healer to waste power healing someone who is in no real jeopardy.
Generally speaking, however, it is best not to pull aggro off the Main Tank in the first place. It makes the Tank's life much easier if you will wait to start attacking a mob until after the Main Tank has generated a substantial amount of threat with the mob. If your class relies heavily on critical strikes to do damage, remember that successive crits will almost certainly draw the mob's aggro onto you. Try to space out your high-damage attacks, allowing the Main Tank to regain the balance of the threat.
As a logical corollary for this, let the Main Tank handle all pulls. If you initiate an attack, the Main Tank may not be prepared to quickly pull the aggro off of you (some threat generating attacks have longer cooldowns that need to reset). In addition, the Main Tank may have a plan involving a line-of-sight pull that requires you to wait behind a wall or column. Pay attention to what kind of pull is planned, and let the Main Tank determine when the encounter should begin.
If you find that you are constantly drawing the aggro off the Main Tank, you may want to consider equipping traits and items that reduce the threat generated by your attacks. If you are a Lore-master in a group where the Guardian is having trouble maintaining aggro, check the Guardian's power. Guardian attacks do very little damage, so they typically do not generate a lot of threat. They need power to be able to produce threat.
Threat is not the only factor that determines aggro. There are certain situations where the mob will ignore part or all of their threat table. For instance, if the character with the highest threat is stunned or mezzed in some way, the mob will move on to the character with the next highest threat (often the Healer).
In addition, some classes have the ability to force a mob to ignore their threat table and attack them for a brief period of time. Since these skills are on fairly long timers, this is usually an emergency response, so when a character uses one of these skills it is important for the Main Tank to quickly use their best threat-generating attacks to quickly reacquire aggro once the skill's effect wears off. This is also a great time for the Minstrel to cast a large group heal, since they do not have to worry about the resulting threat generation.
In large or particularly difficult encounters, an additional character may be assigned to the role of "Off Tank." The Off Tank's job is often to hold the aggro of a second, particularly dangerous mob if the Main Tank would not be able to handle all of the incoming damage. Alternatively, an Off Tank can be used to grab the aggro of any mobs who break off from the Main Tank. This can be particularly handy if the Main Tank needs to recover from a stun or mez.
Related to the concept of aggro management is the principle of coordinated attacks. Generally speaking, everyone in a group should be attacking the same target. There are two reasons for this. The first is that it makes the Main Tank's job much easier, since they only have to worry about constantly escalating the aggro of one mob. In addition, a fellow that takes down one mob at a time from a group of four mobs will take much less damage than the fellow that tries to take down all four mobs at once. For an example of how this might work, consider this scenario:
There is a group of four mobs, each with 6,000 morale. There are four members in your party, each of whom can do 25 damage per second. If each of you were to focus on a different mob, it would take the four of you 4 minutes to kill the mobs, but all four of you would be getting attacked by four mobs the entire time.
On the other hand, if all of you focus your attacks on one target at a time, it would still take you four minutes; but you would only have three mobs attacking you after the first minute, two mobs attacking you after the second, and one mob attacking you after the third. Your outgoing damage would be the same, but your incoming damage would be nearly halved.
With this in mind, you definitely want to focus your fire on one target. The easiest way to do this is to designate one member of the fellow (often the Hunter) as the "Main Assist." The Main Assist's target should always be the target for all the members of the group whose role is to do damage. The Main Assist should not be the Main Tank, because the Main Tank frequently has to change targets to reacquire aggro.
The LotRO user interface has a target assist window designed specifically to aid you in focusing fire on the Main Assist's target. Use it. Alternatively, click the portrait of the Main Assist then press your "assist" key ("F" by default) to acquire that target. Once the Main Tank has generated enough threat, open fire - being sure to balance your attacks to keep from generating a burst of aggro. Once your target is defeated, repeat the process again.
If you are the Main Assist, remember to prioritize your targets. One possible order is:
- Mobs who can heal other mobs
- Mobs who use tactical/magic damage
- Mobs who use ranged damage (if they are nearby)
- Mobs who use melee damage
- Mob who use ranged damage (if they are at range)
- Elites (hopefully these will stay mezzed until you are ready for them)
This order focuses on the mobs that can generally be defeated the most quickly and the ones that are the biggest nuisance first.
Remember that higher-level mobs generally have high resistances to common forms of damage. For example, look at the resistances of this Nazgûl from the final chapter of Book V. Choose your weapons and attacks accordingly. Applying the right debuffs can compensate for this by making a target more vulnerable to certain kinds of damage. Know in advance which debuffs your lore-masters and burglars plan to apply, and plan your attacks accordingly.
On a related note, every class in LotRO has the ability to remove a particular category of mob buff called a "corruption." Corruption buffs can be identified by the blue border around their icon under the mob's portrait. Corruptions can mitigate or reflect damage, and can also do a host of other things which make killing the mob more difficult. Removing corruptions is generally the task of support classes like lore-masters or minstrels; but if you see one on your targetted mob and have the ability to remove it - do so.
A highly effective method of managing aggro is "crowd control" - the process of keeping certain mobs out of the fight. The two basic categories of crowd control are mezzes and roots. A "mez" renders a mob unconscious for a set period of time. A "root" immobilizes the mob for a set period of time, but - unlike with a mez - the mob can still use ranged/tactical attacks as well as melee attacks against anyone in range. (They cannot, however, chase after you, which makes rooting very handy for retreats.)
In Lord of the Rings Online, crowd-control is the primary responsibility of the Lore-master. A Lore-master can keep up to two non-undead mob's mezzed indefinitely, since they have a thirty-second mez on a fifteen-second timer. In addition, the Lore-master can root, in-combat, up to three mobs (or up to eight with the Deep Lore trait equipped). Lore-masters can also mez groups of up to three undead for fifteen seconds every minute.
Hunters and Burglars also have crowd control skills. Hunters can place traps, which root up to three mobs at a time. Burglars have a mez that works against a single humanoid, undead, or dragonkin for thirty seconds every minute.
These skills make an excellent argument for the value of these classes in a group, but if the group does not understand how crowd control works the significant benefits offered by these skills will be lost. The most important rule regarding crowd control is: "damage breaks mezzes and roots." If you are a damage class, please write this phrase down and stick it on your monitor.
If there is time, it is helpful for the group leader to use the target marking feature to mark targets for roots and mezzes. If this happens, make sure that you do not attack the marked mobs. It is equally important that you do not use any AE attacks on mobs that are near the marked mobs.
If mobs aren't marked, you will simply need to pay attention. When possible, most players will call out the target of their mezzes and roots. This is easy to do with a target macro. Note the mob they call out, and avoid attacking it, damaging it with a a DoT, or attacking anything near it with an AE attack.
Also note that when a ranged or tactical mob joins the fight in the middle of a battle, characters with crowd control abilities will generally use a mez to take it out of the battle. Do not stop attacking the Main Assist's target just because an Onburz Orc Archer has decided to join the fray. Let the crowd control folks do their job.
A common way for the best-laid crowd control and aggro management plans of a party to go awry is for one of the members of the group to forget about "knockback" attacks. Knockback attacks are typically used by large, elite mobs. A knockback can fling a player's character thirty or forty meters, which can be annoying in and of itself. More significantly, the charcter can be knocked into an entire group of mobs, drawing their aggro onto the entire party. This almost always leads to a wipe. Even when a launched character does not come running back with a party of trolls on their heels, there is still the chance thay they will be lobbed off of a cliff, making it difficult to rejoin the fray. All of this is to say: if you are fighting a mob with a knockback attack, stand with your back to a wall or stay behind the mob.
On the surface, healing in combat seems like a simple matter. When a mob damages a character's morale, the Healer restores that morale. Minstrels and Runekeepers are the primary healers in Lord of the Rings Online. Captains also have a range of healing skills. In a pinch, a Lore-master can also toss the occasional heal to a wounded player.
Since the healer does not have a limitless well of power, things are a little more complicated than simply keeping everyone's morale at maximum. A Healer's job is not to keep your morale at one hundred percent. The Healer's job is to keep you from dying. Consequently, if the healer sees you taking damage, but realizes that the Main Tank is about to take the aggro off of you, they may elect not to heal you since you will not be taking additional damage.
Also, rememeber that if the Healer is taking damage, it is very difficult for them to implement their healing skills. Depending on the circumstance, it may be necessary for the Off-Tank or another sturdy character to take the aggro off the Healer and try and drag the mob back to the Main Tank. Likewise, a Crowd-Control character may need to quickly mez or root the aggroing mob until the Main Tank can re-acquire the aggro.
In addition, the Healer is not the only person responsible for your health. You are expected to take every possible precaution to minimize how much damage you take - even and especially if you are the Main Tank.
The best way to minimize damage is to avoid it. A character with a shield has three chances to avoid an incoming attack: they can parry it, evade it, or block it. Characters who anticipate having to tank should raise their percent chance for each as high as possible. Everyone else would do well to keep an eye on those percentages as well.
If you cannot avoid damage, you can try to mitigate it. Armor is the primary mitigation method in Lord of the Rings Online, but it only mitigates incoming "common" damage. This is not as helpful as it may seem, since at the highest levels "common" damage is, in fact, quite uncommon. Nevertheless, an excellent armor rating can make the difference in a close fight.
Incoming damage can also be signficantly reduced by stacking the right combination of debuffs on a mob. Debuffing is the primary responsibility of lore-masters and burglars, and it is a good idea for them to plan their debuffing strategy in advance to maximize its efficiency. Having two characters apply the same (or two mutually exclusive) debuffs is a waste of time and power.
Should you draw the ire of an attacking mob, do not run around haphazardly like a scalded hobbit who grabbed a pie without oven mitts. If you are near the Main Tank, simply stop attacking your mob and wait for the Main Tank to re-acquire aggro. Otherwise, try dragging the mob back to the Main Tank. Whatever you do, do not force the Main Tank to have to run around after you trying to grab the mob off of your back. Not only is it cowardly and embarassing, but sometimes the Main Tank will simply decide it is not worth the risk of losing aggro from all the other mobs.
Standard attacks should not be your only concern. Poison, disease, and wounds can also damage a character. If you know in advance that you are going to be fighting mobs that use one of these kinds of attacks, use items, virtues and spells to minimize your vulnerability to that particular method of damage. Lore-masters have a variety of short buffs that they can apply preventatively as well as curatively to minimize the impact of wounds and diseases. Hunters and Burglars can do the same for poisions, and Captains can remove fear effects. Players with characters who have these abilities should be prepared to use them liberally in tough encounters. The "show dispellable effects only" UI option can help you stay aware of anything that might be about to kill you from within.
In your spare time, work on levelling all of your virtue traits. It is relatively inexpensive to swap them out before a large battle or instanced fight, and at high levels many of them provide considerable protection against certain damage types.
It is generally desireable for your group to stay clustered together. This minimizes the likelihood of one of you straying into an additional mob's aggro radius. It also makes it easier for support characters to heal your DoT's and for the Main Tank to find you to pull aggro off of you. On the other hand, it also makes you a prime target for AE attacks. If you are fighting a mob who uses AE's that only affect mobs in front of them, then the tank needs to keep the mob pointed away from the rest of the group. If the mob's AE attacks radiate out from them in other directions, then fellow members need to take up pre-assigned positions that minimize the impact of these attacks.
Finally, remember that you can and should have potions with you to heal and restore your power in an emergency.
Burglars have the ability to trigger openings for "conjunctions" (also called "fellowship maneuvers"). Even if your fellowship does not have a burglar with them, there is a random chance that, whenever you are fighting an elite or nemesis mob, one of your fellow members will trigger an opening for a conjunction.
If your group takes full advantage of that opportunity, the potential benefits are tremendous. You can heal yourselves, regain your power, and significantly damage the mob you are fighting - all witn one properly-executed conjunction.
Pulling that off, however, requires advance planning. When the conjunction is opened, every member of the fellowship sees four buttons: red, yellow, blue, and green. To unlock a particular conjunction, each member of the group must select the correct color in the correct order. To make things more challenging, certain classes can only select certain colors if they are in melee range of the mob on whom the conjunction was opened.
To avoid any confusion, the fellowship leader should choose which conjunction will be used throughout a particular mission or instance. Experienced fellowships who group together a lot may want to work out a number of conjunction options for various situations; but this is simply too difficult for most groups to master. To make life easier on everyone, the fellowship leader should assign a color and sequence to every member of the group, and then encourage everyone to practice on a couple of easy elite mobs so that everyone is prepared for the major fights to come. The UI allows the fellowship leader to select the planned maneuver, creating a visual template that will appear whenever a conjunction is initiated.
In general, a balanced group benefits the most from conjunctions that restore power and heal over time.
In putting together a fellowship for a difficult encounter, make certain that there is a balance of skills in the group. Assign the roles of Main Tank, Main Assist, Healer, Crowd Control, and DPS in advance, and make certain that everyone in the group understands how those roles function. After you have assmebled your group and assigned roles, make sure that everyone has potions, food, spell components (including travel rations for hunters - porting in and out of an instance can be handy), and any resistance items that might help in a particular setting. Reminding everyone to empty their bags of any non-necessary items and to repair their equipment is also a good idea.
Before the first mob is killed, the fellowship leader should also make the loot policy clear. Generally speaking, a "Need Before Greed" approach is the most fair. The "need" option should only be used by characters who will actually use the item for which they are rolling. If the item is a crafting resource or recipe, that means the character has the appropriate profession. If the item is a weapon or piece of armor, that means the character will actually equip the item. If no one falls into the need category, then everyone who wants the item for an alt or to sell can roll for it by selecting "greed."
After the loot policy has been determined, establish the conjunction order and head out to do great deeds!
If you want a checklist, one might look like this:
- Assign roles of: Main Tank, Main Assist, Healer, Crowd Control, and DPS
- Check Potions
- Check Food
- Check Spell Components (including travel rations)
- Resistance Items
- Empty bags of trash loot and unnecessary items
- Establish Loot Policy
- Determine Conjunction(s) order and roles
Etiquette rules can vary from group to group, but some basic principles are generally universal. Most of those principles center on loot and node harvesting. If there are several people in the group who can harvest a particular node type, they should take turns harvesting those nodes. The same is true for looting ground-spawn quest items. Additionally, no one should begin looting until they are certain that all mobs have been defeated.
Likewise, general courtesy is as useful in virtual worlds as it is in the material one. Be on time, let people know if you need to be away from your keyboard, and and remember not to take the game too seriously. If you are not having fun, then why are you "playing?"
Burglars - Playing a Burglar requires quick thinking and adaptability. Although you have a variety of tools at your disposal, many of them have sufficiently long cool-downs that you want to use them at the optimal time. Keep as many debuffs as possible on the target, and know when the Lore-master needs help with crowd-control. Even more importantly, keep your eyes open for the ideal moment to trigger a conjuction. In addition, keep an eye on the burst damage from your successful crit's. If you pull the aggro onto yourself, do not panic. Pull out some of your evasive tricks, avoid doing more damage, and let the Main Tank pull the aggro off of you.
Captains - If you are playing with a fellow that is unaccustomed to grouping with a Captain, you may want to take some time to let them know what you can and cannot do. A good Captain is constantly aware of the needs of their group. Do they need buffing and restoration? Do they need an Off Tank? Do they need healing? Make peace with the fact that you are not going to lead the group in DPS; and be proud of the fact that, if you do your job well, your group will survive fights that would have otherwise been impossible.
Champions - Make sure that your group understands that when you have Fervour up you will take damage very quickly, and explain that you are most effective when you can keep multiple targets grouped together. Remember, however, that the tactics you use while soloing are generally not as effective in a group. Although you are most effective when you attack multiple mobs, your Main Tank and Healer might not be able to handle that much threat generation, so you may have to provide what DPS you can on one target at a time. In difficult fights, it is better for your DPS output to go down than for you to break mezzes and generate more threat than the group can handle.
Guardians - This role of the Main Tank is perhaps the most thankless in any fellow. No one notices if you do your job well. When you make one tiny mistake, however, and the minstrel dies - everyone notices. Remember that you have two goals - keep all the aggro on yourself and do not die. Do not worry about how much damage you are doing, worry about how much threat you are generating. If you notice that someone's health is going down rapidly (usually a minstrel or a lore-master's), figure out who is damaging them and draw their aggro. If it is a ranged attacker, encourage the lore-master to mez them if possible.
Since Guardians are not casters, it is easy to ignore the importance of power. Don't! Your damage is not adequate to generate threat, and your best threat-generating attacks require power. Also, remember that slow weapons mean that it will take you longer to land multiple threat-generating attacks. Daggers and other fast weapons are ideal for quickly landing multiple threat-generating attacks. When you're certain that you have the mob's aggro, you can switch to a slower weapon with higher dps or better bonuses.
Wardens - A Warden's lot is much like a Guardian's. You must keep the attention of all the hostile mobs, while using a variety of tricks to keep yourself alive. Cooperate with your minstrel to make sure that you know when it's best for you to heal yourself and when it is preferrable for them to heal you. In addition, remember that it takes planning for you to stay at the top of the threat table since you do not have any instant threat-generating attacks. Watch your power consumption, and let the nearest lore-master know if you are low.
Hunters - On the surface, your role seems simple - do as much damage as possible. Any experienced hunter knows that things are actually more complicated. Be sure to choose the appropriate attacks for the particular mobs. Most importantly, manage your aggro generation. If you get a couple of crit's in a row, try an attack that is less likely to crit or stop attacking for a moment so that the Main Tank can recover the mob's ire. In addition, watch your power use. A friendly lore-master will lend you some of theirs if they have any to spare, but that might not be an option. Acquiring gear and virtues that help with in-combat power regeneration should be a priority (after agility, of course). Finally, remember that your AE attacks will break mezzes, so use them with caution and avoid engaging ranged attackers at all if you think there is a chance that the Lore-master or Burglar will mez them.
Lore-masters - As a utility class, lore-masters can fulfill a number of rolls. With a high tactical critical chance, a properly specialized lore-master can do considerable burst damage. Specialized for support, they can provide near-infinite power regeneration, and some healing. Applying and maintaing the best debuffs on the target (while also removing corruptions) also falls under the lore-master's job description. Most importantly, lore-masters can provide crowd-control.
Lore-masters need to choose their mez and root targets carefully, and make certain that their fellows are aware which targets are being taken out of the fight. Keep in mind that roots work best on melee targets, and mezzes are particularly effective against ranged and casting mobs. It is tempting to get caught up in the desire to rain down fire from heaven, but that is not your primary role. Remember that you are a utility class, and your time is best spent monitoring your fellow's power, debuffing, and maintaining crowd-control.
Runekeepers - Know your role in advance and plan for it accordingly.
Minstrels - Triage needs to be your top priority. Avoid the urge to heal someone just because their morale is dropping. Targets with minimal armor and evasion might need a heal earlier than those who are more sturdy, but ideally the primary target of your heals should be the Main Tank. Limiting the number of times you heal minimizes your threat generation and conserves your power - two things that should be high on your priority list. Conversely, learn to recognize when the Main Tank or another character needs as much healing as you can possibly generate. If your group has a Champion, remember that when they have fervour up they will take damage very quickly and will need a lot of additional healing if they draw aggro. Finally, remember that there will be times your job will be to determine who lives and who gets defeated. If you have to prioritize your healing, allow the most expendable members of your fellowship to die. You can bring them back to fight another day...later.
add - An "additional" mob that arrives and initiates its attack while the fellowship is occupied with another mob or group of mobs.
aggro - The status of being the target of a mob's attacks. Many players also use this as a synonym of "threat" (see below).
aggro radius - The range at which a hostile mob will attack a player's character. This changes depending on the difference between the character's and the mob's.
alt - An "alternate" character, one of the other characters on a player's account.
area effect ("AE" or "AoE") - A spell or attack which affects the nearby targets as well as the original target.
avoidance skills - Skills which allow a character to avoid incoming damage. In Lord of the Rings Online, theses skills are parry, evade, and block.
body pull - Stepping into a mob's aggro radius to use your proximity to get the mob to attack you. This is most effective with melee mobs.
buff - A beneficial spell or state.
conjunction / fellowship maneuver - A special attack, triggered either randomly or by a burglar. Conjunctions require fellowship members to click one of four colored buttons in a particular sequence to get a particular combination of beneficial results.
corruption - A dispellable buff on a mob. It can be recognized by the blue border around the buff icon and by the word "corruption" in the icon's tooltip.
critical strike ("crit") - A high-damage version of an attack. Every attack has a percent chance of being a critical strike, and this chance can be raised with certain attributes and buffs.
damage over time ("DoT") - An attack that continues to do damage periodically after the initial hit. A mob with a DoT on it will usually break a mez or root even if no other attacks are made against it.
damage per second ("DPS") - The average damage-per-hit of a weapon, multiplied by the ratio of its speed, is the weapon's damage-per-second. Colloquially, "DPS" is also used to refer to characters whose primary function in a fellowship is to provide heavy and consistent damage to the mob.
direct damage ("DD") - An attack that immediately damages a target.
debuff - A detrimental spell or state.
fellowship - A group of characters who cooperate to accomplish a particular task. This is called a "party" in many roleplaying games.
heal over time ("HoT") - A beneficial spell that gradual improves a character's morale over a period of time.
instance - An encounter that occurs in it's own "instance" specifically for the fellowship that initiated it. Other fellowships that are participating in the same encounter experience it in their own instances. This allows for scripted behavior and ensures that every fellowship gets to experience the full encounter.
knockback - An attack that launched a player's character into the air and a significant distance away from the mob.
line of sight pull (LOS) - Drawing the aggro of a ranged or tactical mob and then moving behind a wall or other obstacle to force them to come closer (since their attacks are only effective if you are in their line of sight).
mez - An attack that "mesmerizes" a mob, taking them completely out of combat for a period of time.
Main Assist - A character, often a Hunter, who determines the target for the fellow's damage.
Main Tank - A character, usually a Guardian, whose function is to hold the aggro of all of the currently attacking mobs.
mitigation - The reduction of incoming damage due to armor or other factors.
mob - Technically any "mobile object" or non-player character, but typically used to refer only to those NPC's that will attack a player character or which a player's character can attack.
Off Tank - The Off Tank is a character whose role is to either hold the aggro of a second, high-damage mob (while the Main Tank holds the primary mob's aggro) or to hold the aggro of any mobs that slip away from the Main Tank. Off Tanks are used primarily in raids or other large encounters.
pull - Initiating an encounter by attacking a mob. This is usually done at range.
root - An attack that immobilizes a mob. It is most effective against mobs that use melee attacks, since casters and ranged mobs can still damage you while they are rooted.
threat - The level of a mob's desire to attack a particular character.
wipe - An encounter that ends with all members of a fellowship dead. If the people you are grouping with cannot take this all-too-common occurence with a sense of humor, find new group-mates.