Onihebi Kyuuai Monogatari

The old man who adopted orphan Chiaki Kuroiwa and raised him as his own has died. Knowing he'll be alone on his birthday tomorrow, Chiaki's feeling depressed when a snake suddenly appears in front of him. The next thing he knows, a monster breaks down the door and comes charging in!! The jock spirit and the seductive snake are both determined to make him theirs, and Chiaki isn't sure he'll be able to survive!!

(Source: Renta!)