Obsessively wielding the power that N'Zoth has granted her, Nexiona has amassed a cult of worshippers who covet the gifts of the Void. From high atop her temple, within the Black Empire, she transforms her most devout followers into unstoppable horrors.
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.
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.
Despair - Vexiona will apply this debuff on her target which will deal moderate amounts of Shadow damage every second for 6 seconds. When this debuff expires it will deal a small amount of Shadow damage and an additional amount based on your missing health to the entire raid, this damage is called 
Shattered Resolve.
Encroaching Shadows - This will apply debuffs on random non-tank players and deal moderate Shadow damage to that target and all players within 9 yards. After 8 seconds the debuff will expire and leave a pool of 
Shadowy Residue on the floor.
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.
Gift of the Void - When the Void Ascendant dies it will drop Gift of the Void on the ground which will give the person who collects it the ability to cast Annihilation once (this will appear as an extra action button on your UI). When you cast Annihilation all of your stacks of Void Corruption will be cleansed.
Cultists
Dark Gateway - This will deal moderate amounts of Shadow damage to the entire raid and spawn a portal in the room. Cultists will spawn through this portal.
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.
Spellbound Ritualists will cast 
Void Bolt on players which can be interrupted.
[Heroic] Sinister Soulcarvers will melee for a low amount of Physical damage, however they will periodically root a target in the raid and teleport to their location and cast 
Spiteful Assault. You are able to stun the cast of Spiteful Assault to interrupt it.
Phase 2 will begin when Vexiona reaches 100 energy.
Vexiona will fly into the air and no longer be able to summon any adds during this phase.
Twilight Decimator - Vexiona will strike one third of the room with fire, therefore the room will be divided into 3 sections as shown in the map below (the naming convention shown is what I personally used during testing).
She will not strike the same area twice, and she will alternate which side she attacks from. It's best to have someone in the raid be vigilant and call out which section is going to be hit.
Note: From my observations during testing she will never go into the same area twice in a row, so it is safe to enter the previously attacked area once it has cleared (which will be just as the next attack begins).
After casting Twilight Decimator three times Vexiona will land and return to Phase 1.
Adds will no longer spawn in this phase, however Vexiona retains all of her other abilities.
Empowered Void Corruption - Any players hit by any ability will have a stack of Void Corruption applied to them.
Terrifying Presence - If you are not near another player your haste will be reduced by 100%.
Heart of Darkness - Vexiona will channel energy around her, when the cast completes the raid will take a large amount of Shadow damage, however can be reduced based on your distance from Vexiona. If you have the Terrifying Presence debuff you will be feared for 8 seconds and gain an additional Void Corruption stack.
[Heroic] 
Desolation - The target of this will be slowed by 50%, have a ring placed around them and will take a large amount of Shadow damage which will also apply 30 stacks of Void Corruption. However, the damage and the Void Corruption stacks are split with all nearby players. You cannot solo Desolation, if it fails to hit at least 3 players it will trigger 
Destructive Rage.
[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
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
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
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
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
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
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