Ekbab + necromancer boss bug w/devouring diadem

Act 2, bone pits boss. Fighting with Devouring Diadem helmet.

If the necromancer is slain first before Ekbab, and the Devouring Diadem consumes the necromancer's body before Ekbab is slain, the quest item will not drop to proceed with the story.

Bug is likely related to the quest item drop coming from the necromancer's corpse specifically, and since the devouring diadem ate it as part of it's effect and having killed the necromancer first, there's now no necromancer corpse to drop the quest item...thus you just don't get one, and only get the Ekbab drops since he died second.