I just noticed two things while running through the end game and I think it's too small I think for a whole Mantis thing.
First, the death message that pops up still mentions the old Mana mechanic, it reads:
" The Oblivion Crystal is gone and you do not have enough mana. Press E to consume a survivor and respawn "
Secondly, if you backtrack through the Overlord Keep through any of Demonaica's rooms after clearing them you still get dialogue from him even though he's not there.