Inma Seiden - The Legend of the Beast of Lust

The Kameyama family and the Otori family are relatives. They are both trying to compete for control over Otori Academy. Violent crimes are happening inside the academy. There are rumors that someone wearing an iron mask is assaulting female students inside school grounds. Senna is attacked by writing tentacles, and is raped by a demon. The man in the iron mask suddenly appear in front of Mao. She tries to attack him, to no avail. She is rescued by Hitomi Kameyama. More horror is awaiting everybody as this shocking story unfolds!

(Source: AniDB)