Oversoul
The Tentacled One
Is it just me, or does this automatically trigger on its own bats? Did WotC screw up again the way they did with Hostage Taker? You know: creature leaves your graveyard somehow, bat comes out, sac bat to Altar of Dementia, bat disappears from your graveyard (because it's a token), Desecrated Tomb triggers, bat comes out, sac bat to Altar of Dementia, bat disappears from your graveyard (because it's a token), Desecrated Tomb triggers, bat comes out...
If they have to issue some post-print erratum to block that loop, then that's got to be embarrassing, right? But if the answer is something like "bats ceasing to exist doesn't count as them leaving the graveyard" then that's going to be the dumbest thing I've ever heard.