Oh, that's cool! Did you happen to run the predecessor to Dead Gods, the Great Modron March? I had an adventure idea that might serve as a sequel to it. While not a published adventure, it might give you some ideas. This idea I had intended to run as the climax of a Planescape campaign Basically, a rogue faction of Guvners or Mathematicians (a sect from the 2e Planescape boxed set Planes of Law that obsess over the gears of Mechanus) have stumbled upon something with the potential power to change all of reality.
They've found a very large and powerful moigno (living equations also from Planes of Law) that represents the never-ending calculation of pi and its non-terminating decimals. Being ultimate devotees of law in all its forms and the gears of Mechanus in particular, many of the Mathematicians formed a cult of sorts around this moigno “god”. The moigno beseeches them to help it finish calculating pi. But of course this is an impossible task- until one of them thinks of a loophole to cheat the laws of reality as we know it. While we cannot calculate completely calculate pi in a world bound with these physical laws, what about in a world where such laws don’t exist? This is what the “creative” Mathematician has pondered. Cue in the Far Realm.
Using a planar breach to the Far Realm, the moigno cult intends to “solve” pi once and for all. Then, the grateful moigno will ascend to full godhood and bestow some of its powers on its "followers". Thereby, the Mathematicians intend to use that power to stop entropy and stop a central gear of Mechanus, but in doing so, they will freeze all of time.
Of course, not everyone in the cult might seek this goal. One of Orcus’ faithful leftover followers in Mechanus from his time as Primus has infiltrated the Mathematician cult. The follower of Orcus might seek to seize the moigno god’s power and bestow it upon his lord. Perhaps to do this he needs the remnants of the Orcussword your player has?
There were two other published Dungeon adventures I recall of that involve a cult. "The Mad God's Key" (Dungeon 114) is probably too-low level for your group, but it involves a cult of Vecna stealing a key that used to belong to Zagig and could open anything. They want to open an old book of Boccob's faith with it. That module was fun (especially the chase scene at the beginning), but the party got wiped out midway by the zombie rat swarm.
The other one is called The Styes (dungeon 121), and it presents a truly despicable port city that is rotting from the inside. An aboleth is raising a fiendish kraken with the mark of Tharizidun in the shallows of the Styes. He's gathered a cult around it and has been feeding sacrifices to it. All this happens under the guise of a serial killer striking the city. I have never played or ran this one, but it has some fairly disgusting characters that might be of inspiration.
