Okay, now what about a final encounter? In the last, say, 10 minutes of the night (60 rounds) something big and nasty should show up. It will probably be the source of the treasure that will bring the party up to level appropriate wealth. (I guess level 27? Assuming they survive, I mean resurrection probably won't be happening in this adventure, because none of them will be a high enough level to start with it, have time to memorize, or very likely, and opportunity to cast it)

I have this idea that he should be unkillable, and that the goal should be to survive the sixty rounds he's in play. Like the final rush in any timed mission in an RTS. Maybe there's away to render him unstoppable for a set period of time?