In my Dwimmermount game, though there are many gods, only three have played much of a role in the campaign to date: Tyche, Typhon, and the mysterious Turms Termax. Of the three, the worship of Turms is unusual in the present day, because it is closely associated with the end of the Thulian empire. The Thulians, you may recall, are the Roman-equivalents of the Dwimmermount campaign world, but they began their historical role as northern barbarians who overthrew the Eldritch empire that preceded their own.
In the early days of their rule, they were highly suspicious of non-divine magic and its practice was highly regulated (which is the origin of the magic-user class's level titles). Given the misuse to which the Eld had put magic, there were few complaints and the Thulians ruled justly, if occasionally brutally, for several centuries. It was into this environment that an enigmatic wizard named Turms first appeared. Along with his female companion Sarana, he traveled the empire, seeking out forbidden magical knowledge and teaching that such knowledge allowed Men to reach their full potential as the true gods of the cosmos. A cult soon grew up around him, one which encouraged the use of magic and alchemy as a means to divinization and denouncing the gods of the empire as false.
Unsurprisingly, the Thulians didn't take kindly to this and persecuted the followers of Turms, eventually capturing and torturing him until, according to legend, he ascended into godhood before their very eyes. True or not, there's no question that the Thulian official attitude toward the cult changed. Termaxians (as his followers became known) were able to practice their faith openly and the restrictions on magic were lifted. Within a generation, the Thulian empire came to depend heavily on both magic and Termaxians, as it spread farther across the continent and tightened its grip over its subjects. Over time, the empire started to treat other religions as it had once treated the followers of Turms. Only those faiths who acknowledged their gods as aspects of Turms Termax were allowed to operate unrestricted. Those who resisted were ruthlessly quashed.
Resentment against the Turms-worshiping Thulians grew, especially as they grew ever more tyrannical. Rebellions across the empire eventually tore it asunder and the Thulians -- and Termaxians -- were cast down, to be replaced a welter of new, localized powers. In most regions, Termaxianism was outlawed once again and the common folk speak in hushed tones about its supposedly sinister "secret" doctrines. Even those with an inkling of the truth nevertheless show little affection for Turms and his faith has dwindled into comparative obscurity, except among magic-users.
Turms Termax is one of the "great mysteries" of the Dwimmermount campaign. I created the god to be a useful MacGuffin in both the megadungeon and other adventures. As you can see, he's an amalgam of Hermes Trismegistus, Simon Magus, and certain of the Roman emperors. He gave me a good way to introduce secret societies into the campaign (like the Argent Twilight), employ alchemical symbolism (like azoth), and open up the question about who and what they gods really are. So far Turms has proven very successful in these goals and he's one of my favorite recent creations, since he gives me an excuse to indulge in esoteric whimsies of various sorts. And finally, it's nice to be able to use a mystery cult in D&D rather than the peculiar kind of ahistorical polytheism that most gaming tends to present.