Here's his abilities:

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Aura of Despair - Prevents most mana regeneration methods. Aspect of the Viper, Judgements of the Wise, Shamanistic Rage, and Spiritual Attunement remain partially effective. The first 3 do not cause Replenishment. Melee attack speed is reduced by 20% for all raid members as well.
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Shadow Crash - Shoots a shadow missile towards a random target (preferring ranged), inflicting 11,310 to 12,690 Shadow damage to the target and anyone in 10 yd radius. A dark puddle spawns under the area of impact, increasing magical damage by 100%, decreasing casting speed by 100%, healing done and mana cost by 75%.
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Mark of the Faceless - Applies a Mark of the Faceless to a random raid member (preferring ranged), causing them to drain 5,000 hit points every 1 second, for 10 seconds, from other nearby players. The health drained heals Vezax.
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Searing Flames - Inflicts 13,875 to 16,125 Fire damage to everybody in the room, and reduces their armor by 75%. Can be interrupted.
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Surge of Darkness - Increases Vezax's damage by 100%, but decreases his speed by 55%, for 10 seconds. Cast every 60-70 seconds.
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Saronite Barrier - Reduces all damage taken by 99%. Cast after Vezax summons the 8th Saronite Vapor. Vezax will also summon a Saronite Animus. The Barrier disappears when the Animus dies.
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Saronite Animus - Casts Profound Darkness frequently, which does 750 Shadow damage to the entire raid and increases the Shadow damage taken by 10%.

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My plan is to have everyone spread out on either side of the room (we'll split people up by group)

Ranged will kill the Saronite Vapors to make the puddles available for ranged casters and healers to stand in.

Spread out to avoid Mark of the Faceless (Get AWAY from this person!) and avoid Shadow Crash until it lands.

Think Loatheb for killing the vapors, and think Hodir for the Shadow Crash. Both of these abilities leave behind puddles that you want to stand in

A note on Saronite Vapors: Tankspot advises only 5-6 stacks. Use this as a guideline, but err on the side of caution. Don't kill yourself to get mana back. These are the green puddles on the ground.

Shadow Crash: is a slow-moving projectile aimed at a target. I don't know if it follows the target or just heads towards their original position (a'la Voidreaver) I think it's the latter. So if you see a big purple ball of death hurtling at you move away. After it lands Ranged dps should stand in this as it gives a stupid amount of awesome buffs.