Credit goes to Maliander of Wowhead.com

Kill General Vezax on the hard mode.

Vezax spawns a total of 8 Saronite Crystals over time. Killing these Saronite Crystals greates Saronite Vapors on the ground, which allow casters standing in them to regain mana to counter the effects of the permanently active Aura of Despair.
If no Crystals are destroyed, eventually 8 Crystals will be alive at the same time. These Crystals then merge to form the the add Saronite Animus (2 million HP). This add casts the stacking debuff Profound Darkness on the raid and Vezax gains the buff Void Barrier as long as this add is alive.

Lack of mana regeneration means that caster DPS without functioning mana-regaining mechanics will have to make extensive use of the Shadow Crash buff and resort to wanding when not buffed.

This achievement requires the raid to kill this add before killing Vezax. Since Vezax is invulnerable as long as the add is alive, this seems like a sensible approach once the Saronite Animus has spawned. Killing any Crystals before the 8th is spawned will make sure the Saronite Animus never spawns, as will killing Vezax before the 8th Crystal has spawned (though it is not likely this is possible).