Tanking: Three tanks (one for each Prince), comprising one ranged tank for Keleseth and a standard tank each for Valanar and Taldaram. Taldaram and Valanar should be tanked in one half of the room at least 13 yards apart. Taldaram should be kept further to the side if possible so that the Empowered Flame Orbs have as long a path as possible.

Buffs: Shadow and Fire Resistance would be handy.

Key Debuffs: Shadow Resonance; Shadow Prison (Heroic only)

HP: 33,500,000 (5,600,000 on 10-man), shared.

Enrage: 10 minutes.

All three of the Princes are engaged together, however only one will be attackable at a time. One Prince will randomly gain Invocation of Blood (although always starting with Valanar), which makes them attackable. The other Princes will only have 1HP and cannot be killed (although you can do full damage for threat purposes, just like a target dummy), so DPS must switch to whichever Prince currently has Invocation of Blood.
When a Prince gains Invocation of Blood they will gain an upgraded version of their standard ability; see later.

When the pull happens, the Valanar and Taldaram tanks should move their Princes off to one side of the room, to give the Keleseth tank room to manoeuvre and the raid room to spread, while remaining 15 yards apart to avoid knockbacks. Be careful to stay 10-15 yards off the wall though or you can become pinned by a Shock Vortex. The Keleseth tank should take up a position in the middle-left of the room and start building aggro.

Keleseth
Keleseth’s part of the fight tends to exist in parallel to the rest of the fight (except when he’s being DPS’d) so we’ll examine him first.

Keleseth can be either ranged-tanked or tanked by someone with moderate mobility and ranged abilities. We typically use a warlock with a pet and tanking spec. Keleseth will simply shoot Shadow Lance at his tank for 17,500 Shadow damage (16,400 damage on 10-man) every 1.5s. When he gains Invocation of Blood he will change to shooting Empowered Shadow Lance, which deals 92,000 Shadow damage (80,000 on 10-man) instead.

The key to surviving this is for the tank to aggro and gather near himself the Dark Nuclei (singular: “Dark Nucleus”) which will spawn periodically around the room. The Nuclei cast Shadow Resonance which deals 1k Shadow damage every 3 seconds to their target but also reduces the shadow damage they take 35%. This damage reduction stacks multiplicatively, so the more Dark Nuclei the tank has channeling on him the less damage he will take. The table below shows the size of normal and Empowered Shadow Lance hits on 10 and 25-man.

Keleseth's damage as a function of the Nuclei on his tank

What this shows is that the tank should aim to have 4 or 5 Dark Nuclei on him at all times, which will occupy the vast majority of their time. If Keleseth gains Invocation of Blood when the tank has only 3 Nuclei it is survivable with focused healing and possible the help of an external cooldown, but this is not ideal. At the beginning of the fight the tank will need focused healing until he can gather one or two Nuclei, after which the healing load is less demanding until Keleseth gains Invocation of Blood.

Extra tips:
– The Nuclei have fairly low HP and will slowly die over time, so the tank needs to be careful of damaging them too much, and will need to keep gathering them all the time.
– A felguard works really nicely for picking up Nuclei. Charge –> follow, rinse, repeat.
– Dark Nuclei reportedly aggro on the last person to hit them.
– If Nuclei spawn far from the tank, a ranged player in the middle of the room can hit them with a low-damage, non-DoT spell to bring them closer to assist the Keleseth tank.

The rest of the raid

For the rest of the raid, there are four other abilities to worry about, two of which will change slightly when Valanar or Taldaram gains Invocation of Blood.

The first, and most interesting, is the Kinetic Bombs. These are spawned at a random points in the room and fall slowly to the floor. If one lands it will explode, dealing 11k damage (10k on 10-man) and knocking back anyone within 50 yards. This is survivable but a distraction, but fortunately it’s easily countered: Any direct damage (DoTs do not work, nor does Mind Flay at the time of writing, sorry Shadow Priests!) causes the Bomb to rise up in the air again. There will be 2 active at once on 10-man and 3 at once on 25-man. One person can keep two Bombs in the air if they’re close together, although if they are too separate a backup person will be needed to help out; it is valuable to ensure you have ranged assigned to sectors of the room in preparation.
Hunters’ pets can be assigned to attack a Kinetic Bomb and they will wait under it and bite it when it is about to land. Some attention is still needed, but this can be an effective way to handle them.

The second normal ability to watch out for is Shock Vortex. This is a white windy swirly animation summoned under a random raid member. It swirls for a few seconds, after which it will begin to pulse. Each pulse will knock anyone within 13 yards out and hit them for 7k physical damage (5k on 10-man). While staying spread out all the time will trivialise this, this is not always possible (melee DPS need to be grouped up, for one obvious example), so you will need to adjust quickly if one ends up too close to you.
If you are within 13 yards of a wall it is possible to be repeatedly knocked back which will rapidly kill you, so be mindful of your position relative to a Vortex.

The third normal ability is Conjure Flame. Taldaram summons a small flame sphere which chases someone, exploding on impact dealing 11,250 Fire damage (7500 on 10-man) to players within 15 yards. Just scatter from its target.

The fourth normal ability is Taldaram’s frontal cone magic DoT called Glittering Sparks which deals 3,900 Fire damage (3,400 on 10-man) every 2 seconds for 8 seconds, and slows movement speed by 20%. This has 90 yard range. He seems to turn to face his random target when he casts this. Just dispel it.

When Invocation of Blood goes onto Valanar or Taldaram, a couple of new abilities come into play:

When Valanar gains Invocation of Blood, he will stop casting Shock Vortex and switch to the 4.5s cast Empowered Shock Vortex which will create a Shock Vortex on every player in the raid which will do the same 13-yard damage and knockback but dissipate instead of staying and pulsing. If multiple players are close to each other then the damage will be multiplied because they hit each other. As soon as you see the cast you should spread out until he has finished casting. Ranged should be aware that melee will need to run out, so ranged should make room for them, and melee will probably need to assign directions, especially on 25-man or heroic, so they don’t all end up running to the same place.
Tip: Warriors can get out very easily here using Intervene.

When Taldaram gains Invocation of Blood, his conjured Flames will become Empowered Flames. This summons a larger ball of flame which chases a player, exploding for >25k (with the same splash characteristics) when it hits its target. However when the Empowered Flame passes within 10 yards of someone en route it reduces its apparent size and final explosion damage, damaging that person a little in the process. Generally the Flame will pass by a handful of people on its way to its target anyway, but a few people should aim to run in its way, especially if it is aimed at the Keleseth tank or his healer(s). The person targeted should run away as well, to maximise the time for it to reduce in power.

The fight gets a bit hectic at times with a lot of different abilities to handle, but once you get a feel for what they do it’s not as bad as it sounds. Stay spread where possible, avoid the white swirls, kite fireballs through the raid and out the other side and switch to the right mob at the right time.

Heroic

• Any movement will put a stacking debuff on you called Shadow Prison, adding a stack every second you spend moving. If you move with the debuff, you take 500 Shadow damage per stack. Note that this is not a DoT, and if you stop moving, you also stop taking damage. Shadow Prison has a 10 second duration. (Note that the Keleseth tank can still move relatively freely, since they should already have high shadow damage reduction.
• Glittering Sparks will inflict 21k damage over 10 seconds and also reduces movement speed by 40% for the duration (Normal: 20% and 12k damage), meaning it needs to be dispelled ASAP, especially on melee who need to run from Empowered Shock Vortex.
• Flame Spheres hit harder, and when Empowered they will probably kill someone if their size is not reduced quite a bit.
• Kinetic Bombs will be summoned closer to the ground and will drop significantly faster. You need a lot more people paying a lot more attention to keeping them afloat. Survival >> DPS on this fight.
  Repositioning after a Bomb explosion will stack Shadow Prison on top of the base explosion damage, so if you are at low health, stop moving and wait for heals first. Remember, Shadow Prison doesn’t hurt if you don’t move.
• Shock Vortexes inflict approximately 50% more damage. More than a double hit can mean a gib, so people must spread out.

Generally the fight runs much the same as normal mode, except with more care taken with positioning. Ranged should be spread out to ensure Kinetic Bombs can be handled quickly. Melee should know where to run out to limit excessive moving and to prevent chaining the Empowered Shock Vortex.
When a Flame Orb travels out it needs to hit a few people on the way past, so people need to chase the Orb a little way. Ranged should stand away from Taldaram so that if they are targeted the Orb has a good, long travel time.