Kuro Ai: Hitoyozumakan Inkou Rannyuuroku
English name: Dark Love
Genres: Hentai
Type: OVA, Episodes: 2
Aired: Apr 29, 2005 to Jun 24, 2005
Studio: Shinkuukan
Source: Visual novel
Rating: Rx - Hentai
Synopsis:
Tetsuya's investigation into a luxurious hotel turns over a nest of yakuza and a bizarre orgy cult. On rich silk sheets and velvet chairs, the rich and powerful participate in sex acts so depraved that just witnessing them is likely to turn a man into a monster. But the true purpose of this cult is a pact with evil energies that can literally turn Tetsuya into an inhuman demon. 

(Source: ANN)