Junketsu Karyuudo

This story takes place in a certain Western town, in an all-girls boarding school for rich girls.

During a midnight walk, Erika hears a strange voice in her head. She is hypnotized and lured into an underground chamber, where she meets a vampire that just woke up from his eternal slumber. He rapes and bites her, turning her into his faithful servant - vampire Eris.

From now on, every night Erika turns into Eris and, for the sake of her master's complete revival, assaults and sheds blood of every virgin at school...

[edited from the official website]