Vexiona

This is a three Phase encounter, with Phases 1 and 2 repeating until she reaches 40% health at which point Phase 3 will begin.

You will want to pay attention to your Void Corruption stacks throughout the encounter and cleanse them with Gift of the Void.

Phase 1 (Grounded)

Vexiona

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.

Void Ascendant

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.

[Heroic] Destructive Rage - If Desolation does not hit at least 3 players, the boss will gain 100% increased damage for 15 seconds and cast Desolation again. 

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.

No Escape - The Iron-Willed Enforcer will periodically grip in the 3 furthest players from it within 50 yards to its location and cast Brutal Smash.

Brutal Smash - This will deal lethal (981k) Physical damage and if you survive will stun you for 4 seconds.

During Phase 3 a Shadow of Vexiona will spawn which will cast Twilight Decimator for the entire phase.

Desolation deals twice as much damage and has 60 stacks of Void Corruption (up from 30)

The tank who starts with Vexiona should have the first Gift of the Void

Swap who is tanking Vexiona after each tank has cleared their stacks of Void Corruption with Gift of the Void

Ensure your health is high and utilise defensive cooldowns/externals/self healing for Despair

Brewmaster Monk

Black Ox Statue is useful to help gather the adds and make sure they do not instantly go for a Healer

Zen Meditation can be used during Twilight Breath to mitigate the damage

Vengeance Demon Hunter

Sigil of Chains is useful here to mass grip adds onto the boss for more efficient DPS

Sigil of Silence can be used to help bring the Spellbound Ritualists into the group

Protection Paladin

A mouseover macro of Avenger's Shield will be useful to keep the Void Bolts interrupted from the Spellbound Ritualists

Blessing of Spellwarding and Divine Shield can be used to negate the damage from Twilight Breath

Protection Warrior

Shattered Resolve can be reflected (with Spell Reflection) onto Vexiona dealing damage to her

Rallying Cry is very useful for the high amounts of damage the raid will be taking in Phase 3

Guardian Druid

Stampeding Roar can be very useful to help people move from Twilight Decimator

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.

Anti-Magic Shell can be used to negate gaining stacks of Annihilation from the Void Ascendant