He spawns after players talk to him, but not before they destroy the dream catcher-like shamanic wards and kill the Spectral Khazra Shaman (who, most likely, was preventing Gharbad from taking over the mummy). Once Gharbad lives again, he gains very fast and damaging melee attack, as well as one random affix (Fast, Mortar, Extra Health). Killing him ends the event.