do mission like you normally would. Get down to Vulak'Aerr. Engage him, wait until you see 6 of the following messages in the event text.
Power flows out from the central gem to the smaller gems. Something forms under one of them.
Kill Vulak'Aerr. Get out of the pit, let the 6 golems explode, go in to pit and click crystal, open chest.
Do not click the crystal before the golems have exploded.