This particular case is an odd one, because while I’d had Belmaphet’s motivations and various schemes in my head and adapting to your actions throughout the entire game (and was conscious of Torm’s thoughts on the matter), the idea to have a trial and for Asmodeus to leap on the opportunity literally only came to me when you arrived back at the Umansionable at the end of the last game. I was laying those tracks as they happened.
I had in no way considered the possibility that Belmaphet would be captured, let alone turned into a chair, and I suddenly realised that Torm would want to speak with him, then just as suddenly realised that Asmodeus would exploit the breaking of his decree against Belmaphet returning home.
This is the absolute fun of being a DM, and it’s a privilege to have players who run with my hairbrained ideas and give as good as they get.
EDIT - I’d envisaged from early in the game that the Big Finale could be Belmaphet finding a way to lead an invasion of Mount Celestia. As always, these things never
quite work out as expected.