The Dwimmermount megadungeon is a massive beast. I’ve played in an online Roll20 Dwimmermount campaign, and it’s on my list of “adventures that I’d like to run someday”. Dwimmermount’s history and background is tightly tied to its campaign setting, so it wouldn’t fit into either my Astonishing ACKS or Affiliation games.
If I were to run Dwimmermount, I’d need to change things, though. The implied setting starts out as pretty close to “vanilla fantasy” which becomes stranger the deeper you delve. I’d prefer to make things a lot weirder from the start, by combining the setting with Operation Unfathomable (a gonzo funhouse mini-campaign) and taking ideas from Devilmount and Anomalous Subsurface Environment.
Campaign Setting: I’d use the Dwimmermount Wilderness Map for the local area, but try to mesh it with some of the Operation Unfathomable locations for the wider world.
The entrance to the Underworld itself might be near Winterburg, north of the Wintertops. I figure Mastodonia would lie north of the Dwimmermount Wilderness map.
Rules System: Labyrinth Lord (Core Rules, with elements from Advanced Edition by referee decree). (Despite my love of ACKS, my Dwimmermount hardcopy is the LL version and I would use the old B/X or Labyrinth Lord ruleset.)
Character Creation: Ability Scores are 4d6 drop lowest, in order. Classes per Labyrinth Lord, but dwarves are a robo-dwarves, elves are called “tieflings”, and halflings are replaced by lumpen-ones (as a compensation class). Non-human classes require referee consent. Magic-Users gain read magic as a class ability (instead of as a spell). Thieves get d6 hit dice. Leather armour is AC 7; studded leather is AC 6.
I’m just going to leave this page here as a placeholder, if the opportunity ever arises to run Unfathomable Dwimmermount.