We just beat Acererak (we think - he was nearly down and teleported away). It was an incredibly epic! Almost every one of our resources were exhausted (all spellcasters had almost no spell slots left, which has never happened before). We had to first fight the Hags, who were incredibly tough with their ability to disappear to another plane when they wanted to. Then we fought the Atropal, who hit us with multiple levels of exhaustion. Then we fought Acererak. All three battles in a row with zero rest. That was not the plan, we had planned to rest, but it didn't work out that way.
Our DM swapped out all of Acererak's spells and some abilities, and played him like a very high intelligence caster (as he is). He nearly destroyed us all, even with 50 temporary hit points renewing every round. He cast fireball as a legendary action after PC turns (up to three a round, not including his own turn), and dispel magic, and counterspell as a reaction. He moved around to make sure he'd be out of line of sight of casters, or forced them to come right to the edge of the lava pit and risk being pulled or pushed in by a spell. He teleported, managing to split our party for a round (He stepped through the transport curtain in the lava room, then teleported back to the hag room and hit us from behind as half our team charged through the curtain after him).
Really epic battle. I think we had been in initiative for 14 or 15 rounds by the end.
