I have been running Ptolus since 2006 under multiple systems (3E, Castles & Crusades and 5E).
Is that endgame dungeon covered in any of the published adventures, or would those convert just fine too? I’ve bandied the thought around for about a year but didn’t want it all to be a huge burden if it’s just be easier to run it in 5e.
It's in the big book, near the end. It's the location atop the spire.
I haven't read all of the side adventures, but the ones I've read have been pretty straightforward. Demon God's Fane is pretty high level, but it's mostly a standard dungeon crawl. (It's not set in Ptolus, but in a town south of there, and the demon lord that's part of the adventure is name-checked in the big book.)
While it's not quite large enough to be called a mega dungeon, Bittermold Keep (Cursed Scrolls #1) took my group the better part of five 4-hour sessions to finally explore all of it. They loved coming back each time and seeing how it had changed, as we play (roughly) in real time between sessions. Start and end each session in a safe haven (usually Wardenwood).
It's important with Shadowdark (and any OSR system) that your adventure design includes encounters which are not always-hostile and/or offer non-combat based solutions to bypass them. I worry with Ptolus originating in the 3e era it will have lots of rooms that are open door -> immediate combat.
You won't regret it, this stuff is fantastic.
Just backed it. I succumb to peer pressure easily.
