Adult PC Game: Beast 21 ~Ryoujoku no Yakata~ aka ~Ryoujoku no Kan~ by birdie soft Archive

10 years ago, the hero suddenly began to be tormented by strange nightmares.

He remembered something about the strange woman in the old mansion from his youth.

Wanting to know the truth, the hero goes with his girlfriend to this mansion on a rainy night.

And then ... HORROR