So, finally, the party encountered and defeated Orcus. The battle was long, and several players went down (but were revived before the end). Orcus tried to flee, but was denied.
I had fun DM'ing this campaign and am still amazed it lasted 2 and a half years. I hope the players had as much fun as I did--it was a published adventure path, and involved a bit of railroading--as well as lots of combat but little non-combat stuff.
On to playing in Promiseland Rising.