Avatar of Hakkar

Sanctum of the Fallen God
Take the western ritual room if the group brought an Egg of Hakkar or Yeh'kinya's Scroll. Clear the room before beginning. During the ritual, collect Hakkari Blood from the Hakkari Bloodkeepers and use it to extinguish the Eternal Flames. Intercept Nightmare Suppressors approaching the altar while the blood carrier completes that job; then prepare for the Avatar rather than starting another pull.
Avatar of Hakkar
Completing the blood-and-flame ritual leads to an Avatar fight where Cause Insanity can turn a faster-moving ally against the group while Lash disables the tank.
Abilities

Cause Insanity
Drives an enemy target temporarily insane, speeding its attacks by 50% and its movement by 80%, as well as causing it to attack its own allies for 6 seconds.
Control the affected ally without killing them; Cause Insanity makes them attack their own group and move faster.

Curse of Tongues
Forces nearby enemies to speak in Demonic, reducing their casting speed by 50% for 15 seconds. Only one curse per warlock can be active on any one target.
Remove Curse of Tongues from the healer first when it slows the casts needed to cover Lash.

Lash
Stuns an enemy for 2 seconds. and has a chance to disarm.
Keep the tank healthy before Lash; the stun and possible disarm prevent an immediate response.
Tank- Keep the Avatar occupied while another player handles an ally affected by Cause Insanity. Plan protection before Lash instead of relying on an ability after the stun lands.
Healer- Cover Lash's stun and possible disarm, and ask for Curse of Tongues to be removed if it slows your casts. Stay clear of an ally turned hostile by Cause Insanity.
Damage- Control a player affected by Cause Insanity instead of attacking them to death. Remove Curse of Tongues from the healer if your class can, then return damage to the Avatar.