In the sinister confines of Castle Nathria, souls are being tortured not to redeem their sins, but to amplify them. The prideful Prince Kael'thas has been burdened with the sins of others, and his pain and hatred are channeled to make him a powerful weapon. This has imperiled his soul like no other, and it must be saved--if only to keep it from being unleashed upon the Shadowlands.
Phase 1 - Swap after 2-3 stacks of 
Vanquished and keep threat on the adds spawning.
Phase 2 - Swap on 2-3 stacks of 
Burning Remnants and avoid being in front of the boss when you are not actively tanking. Ensure the Shade is not inside/near the Phoenixes as they will leave 
Smouldering Remnants behind them.
Blessing of Protection can be used to remove stacks of Vanquished.
Remember to 
Touch of Death the adds, you will get multiple uses of it during the encounter.
Paralysis (reduced duration) and 
Leg Sweep both work on the Soul Infusers to stop them moving for a short period of time. 
Ring of Peace / 
Clash DOES NOT WORK.
Black Ox Statue is a useful pick on this fight to help grab adds.
During add downtime help heal Kael'thas.
Make use of 
Sigil of Silence to interrupt 
Vulgar Brand casts that are away from you.
Sigil of Chains can be used to slow the Soul Infusers.
Sigil of Chains can be used to group the Phoenixes in Phase 2.
Blessing of Protection and 
Divine Shield will remove Vanquished.
Hammer of Justice and 
Blinding Light both work on the Soul Infusers to stop them moving for a short period of time.
Make use of mouseover 
Avenger's Shield macros to interrupt Vulgar Brand casts.
During add downtime help heal Kael'thas.
Blessing of Spellwarding and Divine Shield will prevent Burning Remnants from being applied from the Shade.
You can reset the cooldown of your 
Taunt with 
Heroic Leap should you need multiple.
Shockwave will stun the Soul Infusers to buy a small amount of time for the raid.
Spell Reflection will deflect 
Ember Blast.
Typhoon (Balance Affinity) or 
Ursol's Vortex (Restoration Affinity) both work on the Soul Infusers to push them back and buy a small amount of time for the raid.
During add downtime help heal Kael'thas.
Ursol's Vortex (Restoration Affinity) is very useful to control the Phoenixes in Phase 2.
Chains of Ice is a very powerful slow and should be applied on the Soul Infusers.
Asphyxiate works on the Soul Infusers to stop them moving for a short period of time.
Remember to move near a larger group of adds and 
Blood Boil to gain the maximum benefit from 
Hemostasis.
Anti-Magic Shell will prevent Burning Remnants from being applied from the Shade.
Gorefiend's Grasp (on another Phoenix) can be used to group the Phoenixes in Phase 2.
This is a two phase encounter where you need to heal Kael'thas to full health to defeat the encounter. However at set points in the fight (Phase 2) you will need to deal with the Shade of Kael'thas. Both Kael'thas and his shade use the same health pool but inverse 90% Kael'thas = 10% Shade.
During this phase of the encounter you will have waves of adds to deal with whilst healing Kael'thas. You are also able to interact with the Soul Pedestals near Kael'thas to funnel your own health into him via 
Soul Infusion however this deals increasing amounts of damage per tick and has a 2 minute cooldown via 
Drained Soul.
High Torturer Darithos
This mob is only present at the very start of the encounter and will channel 
Greater Castigation which will deal ticking Shadow damage to multiple players in the raid. The players targeted by the channel will also deal the damage to people within 6 yards of them.
Throughout the Phase 1 waves of the below adds will spawn on set timers:
This add will spawn in the centre of the room and will usually spawn on its own.
Vanquishing Strike - This is the main tank swap mechanic in Phase 1. When this ability is cast it will deal a large amount of Physical damage and apply a stack of Vanquished.
Concussive Smash - This will deal raid wide damage and after each cast the mob will be buffed with 
Reverberation which increases the damage of this by 25% per stack.
These adds will spawn in groups and arrive from the sides of the room.
Crimson Flurry - Several players will be marked and shortly after being marked the assassin will teleport to them and deal an 8 yard AoE which deals a large amount of Physical damage and this adds a bleed over 10 seconds.
Return to Stone - When this triggers be sure to kill the assassin before the cast is finished else they will heal back to full.
These adds will spawn in groups and arrive from the entrance of the room.
Carrying Essence Font - These adds will be carrying an Essence Font on them which will be dropped when they die. If you kill them before they have fully depleted the Essence Font the raid can make use of it.
The Essence Fonts will appear on the boss frames when they have been dropped and will slowly lose health via 
Cracked Reservoir. When they are healed to full they will trigger 
Essence Overflow which will heal Kael'thas by 1%.
Vulgar Brand - If this cast goes through the entire raid will get a debuff for 15 seconds which increases Shadow damage taken by 35%. Interrupt it!
Scornful Blast - Spammable cast, deals a low amount of Shadow damage to its current target.
Door of Shadows - Same as the Venthyr ability, the mob will attempt to teleport away.
These adds will spawn in large groups and arrive in the middle of the room.
Shattering Ruby - This is the main ability from this mob and it will deal a Small amount of Physical damage to all players within 3 yards of its target.
[Heroic] 
Fragmentation - When they die they will explode dealing a moderate amount of Physical damage to all players within 7 yards. As these mobs spawn in large waves you will need to be careful to not kill too many at once.
These mobs spawn in two groups either side of the entrance of the room and run towards Kael'thas.
Soul Infusion - When they reach Kael'thas, they will channel this spell into him dealing 1% of his maximum health in damage to him every 2 seconds! (If the Shade is active they will heal it instead)
Crowd Control these mobs when they spawn to prevent them from reaching Kael'thas
Infusers Boon - When a Soul Infuser dies they will leave this behind which when collected increases the healing of that player by 50% for 14 seconds.
Healers should have a strategy set as to who is getting these buffs to make best use of throughput and cooldowns.
When Kael'thas reaches 40% or 90% he will become immune from all healing and the Shade will spawn. Ideally the raid should be close to the boss to avoid his mechanics easier.
In true Kael'thas style, two Reborn Phoenix adds will spawn as he enters. When these die they will reanimate so there is no pressure to kill these off. They will fixate random players, however the strategy here should be to simply keep them slowed and kite them around the encounter space. As the Phoenixes move they will deal damage to all players within 6 yards of them via 
Smouldering Plumage and leave behind Smouldering Remnants.
Fiery Strike - This is the main tank mechanic of the phase where he will strike the tank (and all players in 12 yards in front) with a moderate amount of Fire damage and leave the Burning Remnants debuff. You will aim to swap at 2-3 stacks as the debuff expires, and never point him to the raid.
Ember Blast - A random non-tank player is targeted with this debuff and after 5 seconds it will detonate dealing a lethal amount of Fire damage and spawn a Smouldering Remnants patch. However this damage can be split with other players within 6 yards. When players are hit with this they will receive a share of the 6 
Lingering Ember debuffs which is a small ticking Fire damage DoT (the debuffs are divided evenly with the amount of people that split the damage).
Blazing Surge - When the Shade reaches full energy he will target a player and deal a large amount of Fire damage to all players in a 40 yard cone aimed at that direction.
[Heroic] This will now target a random players position rather than the position of the player it targeted, therefore to make the whole phase easier you should stand as a group on the boss and move around as you kite the Phoenixes.
This ability does not follow the player, it simply picks the location and casts on that spot so you can side step to avoid it.
This phase ends when you have dealt 10% damage to the Shade.
There are two main changes on Mythic.
Cloak of Flames - This is a new ability in Phase one and this will create a large healing shield on Kael'thas and triggers a 6 second cast of 
Unleashed Pyroclasm. If this cast is successful then it will cause a raid wipe. To interrupt the cast you must heal off the Cloak of Flames shield.
Reborn Phoenixes do not despawn when the Shade disappears.
This section will be updated once a strategy has been devised for Mythic on live servers
Damage intake on this fight all depends on how many add waves you get, the above is an average of a sample of logs during PTR testing.
Swing Timer Boss: 2s (Curse of Weakness 2.4s) - Shade of Kael'thas casts a lot so swings are infrequent.
Blessing of Protection can be used to remove stacks of Vanquished.
Remember to Touch of Death the adds, you will get multiple uses of it during the encounter.
Paralysis (reduced duration) and Leg Sweep both work on the Soul Infusers to stop them moving for a short period of time. Ring of Peace / Clash DOES NOT WORK.
Black Ox Statue is a useful pick on this fight to help grab adds.
During add downtime help heal Kael'thas.
Make use of Sigil of Silence to interrupt Vulgar Brand casts that are away from you.
Sigil of Chains can be used to slow the Soul Infusers.
Sigil of Chains can be used to group the Phoenixes in Phase 2.
Blessing of Protection and Divine Shield will remove Vanquished.
Hammer of Justice and Blinding Light both work on the Soul Infusers to stop them moving for a short period of time.
Make use of mouseover Avenger's Shield macros to interrupt Vulgar Brand casts.
During add downtime help heal Kael'thas.
Blessing of Spellwarding and Divine Shield will prevent Burning Remnants from being applied from the Shade.
You can reset the cooldown of your Taunt with Heroic Leap should you need multiple.
Shockwave will stun the Soul Infusers to buy a small amount of time for the raid.
Spell Reflection will deflect Ember Blast.
Typhoon (Balance Affinity) or Ursol's Vortex (Restoration Affinity) both work on the Soul Infusers to push them back and buy a small amount of time for the raid.
During add downtime help heal Kael'thas.
Ursol's Vortex (Restoration Affinity) is very useful to control the Phoenixes in Phase 2.
Chains of Ice is a very powerful slow and should be applied on the Soul Infusers.
Asphyxiate works on the Soul Infusers to stop them moving for a short period of time.
Remember to move near a larger group of adds and Blood Boil to gain the maximum benefit from Hemostasis.
Anti-Magic Shell will prevent Burning Remnants from being applied from the Shade.