This is a three Phase encounter, with Phases 1 and 2 repeating until she reaches 40% health at which point Phase 3 will begin.
Phase 1 (Grounded)
Twilight Breath - This is a typical dragon breath which will attack all players in a cone in front of the boss. The breath will deal moderate amounts of Shadow damage every 0.4 seconds for 1.6 seconds. Each tick will also apply a stack of Void Corruption.
The Shadowy Residue will persist for the entire encounter, so it is vital that you place them away from locations the raid plans to use. If you have a personal you are able to stack these on top of each other.
Annihilation - This is the iconic ability of this enemy and will channel a beam towards its target. This beam will deal Shadow damage to players and Cultists every 0.25 seconds for 5 seconds. Each hit will apply a debuff which will increase damage taken from Annihilation by 25% for 6 seconds which can stack up to 20 times.
Fanatical Cultists will melee for a low amount of Physical damage, however when they reach 100 energy they will cast Fanatical Ascension. If this cast completes they will transform into a Void Ascendant.
During Phase 1 and 2 an additional add will now spawn the Iron-Willed Enforcer who has the following abilities:
Iron Will - This add is immune to all forms of Crowd Control.
Brutal Smash - This will deal lethal (981k) Physical damage and if you survive will stun you for 4 seconds.
Desolation deals twice as much damage and has 60 stacks of Void Corruption (up from 30)
Tank Damage Profile
The tank who starts with Vexiona should have the first Gift of the Void
Ensure your health is high and utilise defensive cooldowns/externals/self healing for Despair
Black Ox Statue is useful to help gather the adds and make sure they do not instantly go for a Healer
Vengeance Demon Hunter
Sigil of Chains is useful here to mass grip adds onto the boss for more efficient DPS
Rallying Cry is very useful for the high amounts of damage the raid will be taking in Phase 3
Ursol's Vortex can be useful to keep the adds from running away once they have been gathered
Blood Death Knight
Gorefiend's Grasp is useful here to mass grip adds onto the boss for more efficient DPS
Priority adds can be Death Gripped onto the boss for more efficient DPS
Grips and Stuns can be utilised to stop Spiteful Assault from the Soulcarvers.