Perdeen
-- Requires access to the expert rift "Redemption of Perdeen."
-- Uses "Husk: Redemption of Perdeen" from Grim Disciples. Can be purchased for 250 Planarite and Decorated faction status.
-- Husk requires "Malformed Souls" which appear to be from Expert dungeons.
-- Expert Rift appears to be doable with ~6-7 people. Use adds for Transmogrify targets for Spell Haste. Make sure standard roles (Tank, Healer) are fulfilled in party.
This is correct. I am guessing that you need to find Perdeen in order to kill Zerus. I was able to complete that quest today by finding a group doing the Perdeen rift, and helping kill him even though I was not in the actual rift group.