Kyuuketsuki

Kyuuketsuki is a 2 episode hentai anime produced by Valkyria. It is about a girl who is sold to a vampire and trained to be a slave anally, all the while believing that she will be freed once she completes her training.
From MAL:

Synopsis The vampire. A legendary creature that lives in the shadows and preys on humanity. Onohara Mikage is an unfortunate girl who sold herself to a clan of vampires to pay off her parents' debt. What has the clan head, Claude, prepared for her? Will she be able to pay herself off and obtain her freedom? From ANN: Based on: "Kyūketsuki ～Anal Hime Kōkan Chōkyō Roku～" (erotic game) by Valkyria