When I get there, I'm greeted by his 4 original minions popping up out of the ground and just owning me. They were still immune to everything, and the boss was already dead so I don't have any way to kill them.
I'm just happy this wasn't on hardcore, because that would've been a terribly stupid way to die.
Just had to update, it was a Sand Dweller champion with dessicated imp minions. I don't know if that matters at all, I doubt it does, but who knows?