Summum?

This is almost too easy to slot in here:
Article on the Summum cult

While I'm sure that in reality these are some fairly harmless folks, they're a natural for adventuring.

A pyramid shaped temple is erected in town. They quietly begin recruiting, especially among the homeless. Minor crime in the city drops through the floor.

Rumors abound of weird stuff in the temple, including mummification. Then a formerly homeless congregant escapes and runs into a party member. They convey a tale of horro, human sacrifice, forced mumification and ritual torture.

It turns out that the head of the cult id an animated mummy of an ancient magician. They seek to create an army of the undead to take over the city and eventually the country. Can our heroes stop them?