He stands in the back of the arena pit, and is script-attached to Vyemm, but they are not a linked encounter.
Alzid Prime will continually back to 33% HP when taken down to 13% HP unless Vyemm is dead.
Alzid can be seperated from Vyemm via a Coercer's Amnesia spell or by using a pet pull of Alzid up the long set of stairs while the raid pulls vyemm. Alzid will reset and not aggro the raid.