A Cultist Headquarter

Publisher's blurb: Your investigators will probably want to infiltrate this lair, which is run by the grand master of ceremonies. Good luck to them! It's swarming with worshippers, the smell is pestilential because of the corpses of prisoners abandoned in the jails or in the torture room (probably former investigators...). The library is full of ancient books with abominable engravings, the lab with sordid artifacts. But the worst is this big room at the end, protected by a kind of dark magic. They all enter during the moonless nights, and terrifying screams escape from it...