Background: Ever since the Faction Champions encounter was released, I have been making it a particular project to understand and codify the AI behind these foes, in order to better perfect the process of controlling this seemingly uncontrollable encounter. After many weeks or pouring over frapses, parses, and testing individual concepts on my own each week.

While I am still attempting to work out the exact formula for their threat determination, the exact numbers are by and large unimportant. The important part are the general concepts behind the way their threat functions.

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General Concepts

The first important point to understand about faction champions is that nothing is random, as much as it may seem so upon first learning the fight. Every action the mobs take is governed by very specific rules, all of which may be controlled to your favor by understanding the priority system behind their AI.

The most important principle to understand is that all the Faction Champions are governed by threat. It is often said that they have no threat table, but this is a myth. You can even see their threat table on omen when you target one of the mobs.

How is this testable?
Look at omen while targeting a Faction Champion. Observe the fact that threat drop abilities such as Feign Death or Fade work as normal on Faction Champions, even on Heroic Difficulty. Observe the fact that taunt effects function as normal on Normal Difficulty, indicating that threat rules exist and can be manipulated using the normal tools, even if they function according to unusual rules.

The cause of the misunderstanding that Faction Champions have no threat is the fact that their threat is not the result of damage taken or healing sensed. It is calculated by different rules than any other encounter in the game.

These are those rules.

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Melee DPS Champions

Melee DPS Faction Champions calculate their threat based on an additive combination of 3 factors. Each of the three appear to share equal priority for determining target, and the threat list constantly updates.
These factors are ARMOR, PROXIMITY, and HEALTH DEFICIT.

ARMOR
Armor is the first factor of targeting priority and refers to the exact armor value displayed on your character tab. The lower the absolute value of armor on a potential target, the higher the threat.
LOWER ARMOR = HIGHER THREAT

How is this testable?
Have two warlocks stand exactly on top of each other. Have them both activate the spell Fel Armor. Warlocks are one of the highest priority targets in general for Melee when all other factors are equal because they have one of the lowest relative armor values. At some point, one of the melee will be almost guaranteed to target and attempt to attack one of the warlocks wearing Fel Armor. When that happens, have the warlock who was targeted activate Demon Armor, which grants greatly increased additional armor value. Unless outside factors interfere, the melee DPS Faction Champion should always immediately switch targets to the other lock standing in the same spot as the first one, due to the lower armor value.

How does this knowledge help me?
Depending on the class, the understanding of Armor Value's contribution to the aggro calculation can be exploited in various ways. One of the most universal is the ability to have your more vulnerable raid members pre-pot an Indestructible Potion right before entering combat, granting them an additional 2500 armor and thus a much lower threat signature. Similarly, a player who finds themselves vulnerable and unable to escape a melee DPS attacking them may re-actively use an Indestructible Potion to help them drop threat.
Tanks attempting to pick up a target or hold it may wish to put on DPS gear or remove their shield. Players with access to some form of Armor increasing effect such as Inner Fire should pay particular attention to keeping it active at full strength unless they wish to deliberately cause Faction Champions to attack them. Another example is a druid shape-shifting into a higher armor form if focused, such as a feral druid shifting from cat to bear. Warlocks are the class most frequently focus-fired by melee DPS Faction Champions, and their primary threat-signature tool, Demon Armor, was mentioned in the example above.


PROXIMITY
The second piece to the melee DPS Faction Champion threat calculation is proximity; the distance from the Faction Champion to the player. The shorter the distance, the higher the threat.
CLOSER PROXIMITY = HIGHER THREAT

How is this testable?
Have a deathknight Death Grip a melee DPS Faction Champion far away from everyone else in the raid. Then Stun or otherwise CC that Faction Champion. Have any player stand right next to the CC'd Faction Champion, and have everyone else move far, far away. Unless significant outside factors interfere, the Faction Champions will always attempt to attack the player right next to them. For testing purposes, use a class with fairly low relative armor to ensure that the armor and health deficit threat signature of someone else does not override the testing player's proximity threat signature.

How does this knowledge help me?
It doesn't really, since the proper way of handling this knowledge is intuitive already. Melee DPS Faction Champions can only attack you from melee range, and thus it is natural to want to move away from them if they are attacking you. This applies to melee DPS attacking the faction champion too though. It is good practice for a melee DPS who is targeted by a Faction Champion they are DPSing to run away from the Faction Champion and cause it to target switch or to be ineffective at causing any damage until someone else's proximity threat signature overrides theirs and they begin attempting to attack someone else. Reducing as much damage taken by the raid as possible is a primary focus of the entire raid, as the less damage that goes out, the more the healers can focus on dispelling, which results in a far more stable control over the encounter. It is also advisable to keep various forms of snares and movement inhibiting debuffs on melee DPS Faction Champions as often as possible.


HEALTH DEFICIT
The third and final component of a player's threat signature for a melee DPS Faction Champion is their threat deficit. This is defined in terms of the absolute value of hp that player is below their maximum hp. This is the factor which causes the seemingly random "insta-gibs" where multiple Faction Champions all target the same player at once and deliver a devastating combination of attacks. This generally occurs when someone takes a DoT tick or a low armor value player is clipped by an aoe effect like Bladestorm. It gives the illusion of intelligence by causing them to attempt to target whoever is weakest in your raid.
HIGHER HEALTH DEFICIT (more damage taken) = HIGHER THREAT

How is this testable?
Have a deathknight Death Grip a melee DPS Faction Champion far away from the rest of the raid. Then Stun it or otherwise CC it. Have two warlocks or roughly equal armor value stand directly on top of the CC'd Faction Champion. Have one of them Life Tap to half health or lower to create a great enough health deficit threat signature to override any minor differences in armor value threat signature between the two warlocks. When the Faction Champion comes out of CC, unless outside factors interfere it will always attempt to attack the warlock at lower health due to the health deficit from Life Tapping.

How does this knowledge help me?
The biggest application of this aspect of the threat signature is focusing a large amount of attention to defensively dispelling DoT and Debuff effects from your raid. The better your raid's dispelling, the more controllable the fight will be because there will be no random health deficits all over the place causing unpredictable target switching to occur. DoTs tick for hard enough to be a significant danger to raid members by setting them up for an insta-gib situation where their health deficit threat signature causes multiple Faction Champions to all switch targets to them at once.
The other aspect of this knowledge that can be exploited is that if you are a tank attempting to control a mob, and you can get a melee DPS Faction Champion far enough away from the rest of the raid that you have a proximity threat signature advantage to compensate for your lower armor value threat signature, if the Faction Champion starts attacking you it will tend to remain attacking you because it will be constantly causing a health deficit on you unless outside factors interfere.
Another application of this applies to warlocks, who should be encouraged to not Life Tap unless positioned a significant distance away from dangerous Faction Champions or receiving immediate healing.


SUMMARY

ARMOR VALUE + PROXIMITY + HEALTH DEFICIT = THREAT SIGNATURE

Note: The rules for melee DPS Faction Champions all also apply to the Hunter and both the Hunter pet and the Warlock pet. Both pets are subject to taunt even in Heroic Difficulty.


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Ranged DPS Faction Champions

Ranged DPS Faction Champions function on very similar lines to melee DPS Faction Champions, with one difference. Instead of Armor Value being part of their threat calculation, they instead compare Resistance to their school of magic.

RESISTANCE + PROXIMITY + HEALTH DEFICIT = THREAT

RESISTANCE
This aspect of the threat calculation is much less noticeable than the armor value part for melee DPS, since resistance levels tend to be the same for everyone in the raid due to raid wide buffs like Mark of the Wild, Auras, and Totems. Some classes have abilities which grant them higher resistance to magic than normal, and will thus tend to not be targeted by the caster Faction Champions, but in general casters will simply deliver killing blow insta-gibs to raid members already being attacked by melee Faction Champions as health deficit is the primary determining factor in their targeting.

How is this testable?
Stun or otherwise CC a caster Faction Champion. Quickly move the entire raid to the complete opposite side of the room so that they will be out of range of that Faction Champion's attacks. Have two players, one buffed with Mark of the Wild, one without Mark of the Wild, stand directly on top of the CC'ed Faction Champion. When the Faction Champion comes out of CC, unless outside factors interfere, they should always attack the player without Mark of the Wild.

How does this knowledge help me?
If you have players which you find are vulnerable to being focused down by casters, have them use a Flask of Lesser Resistance (+50 all resists), which should help significantly to prevent caster Faction Champions from targeting them unless they have a significant proximity and health deficit threat signature.


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Healers

Healers heal based on a very easily recognizable principle. They place priority on their healing targets based on the lowest absolute hp among friendly targets. What this means is that out of all Faction Champions with a health deficit, no matter how big, the one with the lowest numerical amount of health will be selected as the target of their healing. This is why it is relatively easy to burn down high health targets at first, but when they reach lower health all enemy healers will pour massive amounts of healing into the target and it will be much more difficult to finish off.

How is this testable?
Have one player spend the entire fight DPSing the hunter or warlock pet. Since the pets have much lower absolute health than any of the actual Faction Champions, as long as the one player is constantly doing damage to the pet the healers will focus their primary attention on spamming the pet with healing and HoTs. An enhancement shaman is ideal for this as they can purge off the HoTs. Observe the healing received meters following the engagement.

How does this knowledge help me?
Save your CC diminishing returns on enemy healers for when your kill target is reaching lower levels of health, and then chain them in sequence to solidify a kill. As in the method of testing, assign one player, or alternatively all your raid's pets, to constantly DPS the enemy pets. Be sure not to kill them. All heals directed at these pets your interrupters can simply let go through, and save their shocks and kicks for any heals directed at non-pets. This allows you to do a much more effective job at eliminating healing onto your kill target.

LOWER ABSOLUTE HEALTH = HIGHER HEALING PRIORITY

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Conclusion:
I hope this helps those of you out there either working on this fight for the first time, or working towards ever higher levels of tribute all the way up to Tribute to Immortality.
Knowledge is Power.

regards.

can i get a TL;DR?



WHATEVA WHATEVA THEY DO WHAT THEY WANT.

that armor thing makes so much sense b/c i hardly ever get targeted but my other healers get targeted... druids especially

Excellent write up and analysis, I'd figured out the HP deficit portion of their AI several weeks ago (along with many other people) but couldn't figure out the other factors contributing to their threat table. The combination of HP deficit and proximity is probably what allows them to be tanked, although knowing about the armor aspect of their threat table is certainly useful information for priests/locks/druids who get focused. Thanks!

im impressed, very thorough job, OP

I can now give a 20 minute dialogue to the warlocks when they say "gawd why do i always die on this fight"

so you're saying our warlocks should drink ironshield potions. got it.

I have my own observations on the melee mobs, namely they seem to be tempted to 'communicate' targets. in heroic 25man at least, where taunts don't affect their behavior, you'll often find all 3 targetting a tank. typically its a warrior but I've succesfully held all 3 on me in the past.

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Q u o t e:
so you're saying our warlocks should drink ironshield potions. got it.

I have my own observations on the melee mobs, namely they seem to be tempted to 'communicate' targets. in heroic 25man at least, where taunts don't affect their behavior, you'll often find all 3 targetting a tank. typically its a warrior but I've succesfully held all 3 on me in the past.

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If you succeed in getting one target to attack you you are increasing your threat signature via Health Deficit, which in combination of your Proximity threat signature resulting in more of them attempting to atttack you too.
Odds are that most of the low armor targets in your raid were relative distant from from you at the time as well.

Your example is perfectly reasonable according to the rules for threat set down, with no need for any kind of hidden sinister "lets all get that guy muahahaha" mechanic to help at all.

Chances are, all that happened was you were standing right next to one of them, attempting to tank it, and a DoT ticked.

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Q u o t e:

Chances are, all that happened was you were standing right next to one of them, attempting to tank it, and a DoT ticked.

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or better yet i threw the shield and built initial threat.

this topic explains why if i open with high enough threat and keep generating they'll stay on me but if one walks away i'm having such a hard time recovering them. well played. :)

I guess I'll tell my healers to have me no higher than 50% and see how it goes.

just curious, have you tested the use of defensive cooldowns as an aggro dump? being a shaman i don't really have any to speak of ><

PS, GS, Dispersion, Deterrance, DP, Shield Wall, Icebound etc.

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Q u o t e:
just curious, have you tested the use of defensive cooldowns as an aggro dump? being a shaman i don't really have any to speak of ><

PS, GS, Dispersion, Deterrance, DP, Shield Wall, Icebound etc.

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Defensive cooldowns don't have an effect on threat unless they are full immunities like Ice Block or Divine Shield. They simply reduce incoming damage. Still worth using of course, but they won't force a target switch by themselves.

Hand of Salvation TECHNICALLY works, but it doesn't work fast enough for it to really accomplish anything meaningful.

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Q u o t e:
so you're saying our warlocks should drink ironshield potions. got it.

I have my own observations on the melee mobs, namely they seem to be tempted to 'communicate' targets. in heroic 25man at least, where taunts don't affect their behavior, you'll often find all 3 targetting a tank. typically its a warrior but I've succesfully held all 3 on me in the past.

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It can be any tank, I've successfully tanked their melee for large portions of the fight on multiple kills, same thing with our paladin. Once we discovered the whole percentage HP threat mechanic we started sending our pally tank and myself into the fight with low HP (just swap to DPS gear and back) and tell the healers to avoid topping them off. This weeks kill featured our pally holding all the melee up until the point where he died (about two minutes in after two of their healers were down) at which point we rezzed him and he got aggro again since the healer made sure not to top him off. Made for quite an easy fight.

This also probably explains why I rarely am early to die on this fight if I'm running resto. I am always at range and have relatively high armour compared to cloth. Coupled with lots of CC options, it's a good fight to be resto in.

Our poor priests :(

I have also noticed, the faction champs will go after a person who has popped all of their offensive cooldown. I noticed this because everytime I pop all my cooldowns all at once, I get targeted immediately.

You can try testing this.
Have 2 arcane mages stand side by side. Have only one of them pop all their cooldowns (Icy veins, etc etc.). You may notice the faction champs going after the mage that have popped his cooldowns.

hrm I was ready to dispute op but he might be right about the armor thing. I'm normally on the DK in heroic champs and he always targets 2-3 people the entire fight. It's primarily warlocks, and an occasional mage. Checked their armory and they do have the lowest armor values,

I wonder if Hand of Salvation and other aggro-reducing abilities affects the aggro from these sources.

Useful things get posted on RnD only about every 6 months or so... this is it, well done.

Great post and thank you...

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Q u o t e:
I wonder if Hand of Salvation and other aggro-reducing abilities affects the aggro from these sources.

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Hand of Salvation TECHNICALLY works, it just doesnt work fast enough to do any good or be worth casting. It can't even ensure a target-swap on it's own. Its generally just better to counteract the source of the target's high threat signature (heal to full->run away->get more armor) than trying to artificially reduce their threat unless you have some source of instantly dropping it such as shadowmeld or fade.