Sometimes killing your foe just isn't enough. I wonder if the Hive Lord got into the Cultist's body through some parasite, or whether the HL's body was just a vessel for its consciousness. Delightfully Grimdark.
I ran out of space to explain more clearly--it wasn't actually the Hive Lord the second time, it was a group of Shadow Hunters that stayed behind and gradually learned how to take over the base.
