In the depths of the forest of the Lake of Sorrow, just south of Moonville, is an abandoned temple that's been taken over by a cult who worships Numinous Belphegor. The cult site has 3 levels: the upper level, the main level, and the lower level. The upper level is heavily guarded as it's the main entrance to the site. Cultists can be seen engaging in various sexual practices and rituals on the main level.
The cult site has an active portal to and from the Tomb on the Isle of Prisoners on the lower level, but it is not initially accessible until the Princess first uses the other side of the portal on the third level of the Tomb.
According to Jachan's Diary the site served sometimes long ago as a temple for a nameless entity, that is about death and undeath (possibly the Nameless One), but was later abandoned, and subsequently found by Jachan who turned it into a site dedicated to Belphegor.