A skull festooned catacomb with a central chamber that has an ornate, demonic well/ portal to the abyss... Or the Cavern lair of an insane hillbilly cannibal that is also a Cthulhu mythos cultist*...Or* a Satanic church interior with various blasphemous accoutrements, dominated by a hellish stained glass window depicting damnation motifs.

(It might be a challenge to inject some light-heartedness into the above themes, though).