The Restless Cabal
Disclaimer: This fight uses the default taunt rule set, which means that a duration of 20 seconds with no casts of taunt is required for the Diminishing Returns to reset on it. Therefore you will need to wait 20 seconds between taunt casts (if you do not do this 5 times, the boss will be taunt immune)
There are two bosses in this encounter: Zaxasj the Speaker (Ranged), and Fa'thuul the Feared (Melee).
At 75%, 50%, and 25% the boss will activate the nearest relic to them. Each boss should only empower each relic once (3 relics each, 6 relics total), when they activate a relic they gain a 10% damage buff (this stacks up to 3 times). If they activate the same relic twice in a row they will gain a 300% damage increase buff called Power Overwhelming.
These bosses do not share health and will need to be killed at a similar time. When one of them reaches 1 health it will attempt to cast Pact of the Restless, which if completed will heal the boss back to 20%. Get both bosses to 1 health to interrupt the cast and end the encounter.
Zaxasj the Speaker
You will ideally want a ranged DPS player to be tanking this mob.
The main ability for this boss is Aphotic Blast - This will deal less damage the further away the target is from the boss, and leave a debuff on the target and anyone within 5 yards.
This debuff will increase threat generation by 150%
[Heroic] When the Aphotic Blast debuff expires it will transform that player into an Agent of Demise. Once a player is an Agent of Demise they will received reduced healing by 75% and they will be hostile* to the raid.
Cerebral Assault - Zaxasj will face in the direction of a random raid member and begin the 6 second long cast. The cast is in narrow cone from Zaxasj to its target. Players will need to move out of this cone before the cast ends, if they are not able to move out of the cone they will take shadow damage and be hostile* to the raid for 4 seconds.
Crushing Doubt - Two players will be marked and receive a 12 second debuff. During this debuff they are slowed by 25% and will take increasing amounts of damage until the debuff expires at which point it will deal Shadow damage to the raid which is based on your distance from that player.
When Fa'thuul activates a relic it will cast Eldritch Revelation which will spawn three Eldritch Abomination Adds.
These adds will all cast Witness the End. This is a 9 second cast which when complete will cause the add to die and deal Shadow damage to the raid.
You will want to stagger these deaths by interrupting, silencing or stunning the adds.
Interrupt Add 2 + 3 - Add 1 dies
Interrupt Add 3 - Add 2 dies
Add 3 dies
Note: If you activated the tempest caller, you may want to keep all 3 interrupted until its over to prevent any instant deaths.
[Heroic] You will take 100% more damage from each cast that is successful for 6 seconds, meaning you want to delay this by a minimum of 6 seconds.
The boss who activated this will receive an Umbral Shell
This puts an absorb shield on the boss which needs to be broken, as whilst this absorb shield is active the raid will receive a debuff called Embrace of the Void. This debuff will reduce all healing received by 100%.
Trident of Deep Ocean
Any enemies within this shield will have 75% of their damage taken redirected into the shield, and when the Abyssal Collapse cast is finished any remaining value in the shield will be dealt to the raid in Frost damage.
When this relic is activated Storm of Annihilation will be cast.
Over the next 15 seconds there will be pulsing Nature damage going out to the raid.
If any player goes below 25% health they will have Annihilate cast on them instantly killing them.
Please note: There was no Mythic testing for this boss, so this is from the Dungeon Journal
Agent of Demise healing reduction increased to 100% (up from 75%)
Eldritch Abominations gain a new buff - Incomplete Form - This will make them immune to interrupts after 2 minutes.
Witness the end increases the amount of damage taken from the casts by 500% for 12 seconds (up from 100% for 6 seconds from Heroic)
Each of the relics will create an essence of itself which will persist until they are activated again.
Void Stone - A Void Essence will be created and pull players towards it for 10 seconds. Every 40 seconds it will respawn increasing the power of the pulling effect.
Tempest Caller - A Storm Essence will spawn every 20 seconds. These will deal damage to the nearest player every 2 seconds and this damage will stack if the same player is hit multiple times. If this hits a target below 25% health they will be killed instantly by Annihilate.
Tank Damage Profile
Swap when your Shear Mind stack drops (this should be ~2/3 stacks per tank)
Avoid your ranged tank.
Void Stone - DPS down the Umbral Shell on the active boss. Utilise defensives here as healing received is reduced by 100%.
Trident of the Deep Ocean - Put bosses/adds in the shield and AoE as much as possible.
Tempest Caller - Stay above 25% health
Due to the mechanics of stagger you will be able to take more than the recommended 2/3 stacks (if required)
Leg Sweep will stun the Eldritch Abomination
Vengeance Demon Hunter
Sigil of Silence will apply its blanket silence to the Eldritch Abomination
Blessing of Spellwarding can see high value in this encounter due to the large amounts of mechanics it could in theory remove (example: Placing it on a Healer to mass dispel to negate Wrack damage)
Avenger's Shield will apply its blanket silence to the Eldritch Abomination when you hit them directly
Hammer of Justice will stun the Eldritch Abomination
[Mythic - Untested] Blessing of Protection to run through several Oceanic Essences from Trident to clear up encounter space
You can block Shear Mind
Due to the mechanics of Shield Block/Crit Block you will be able to take more than the recommended 2/3 stacks (if required)
During Tempest Caller is a good time to use Rallying Cry
Stampeding Roar can be useful to help the raid move from numerous mechanics
Have Rage pooled for the tank swap so you can immediately have Ironfur uptime
Blood Death Knight
Grip as many adds as possible inside the Trident bubble to help deplete it
Asphyxiate will stun the Eldritch Abomination
[Mythic] Icebound Fortitude to run through several Oceanic Essences from the Trident to clear up encounter space
[Mythic] Anti-Magic Shell to run through several Oceanic Essences from the Trident to clear up encounter space