What's the name of this anime?

NOTE: I have forgot a lot of details (including the name obviously, so sorry it's vague)
Okay so this teenage girl gets chased kwa an Oni (like a vampire thing) and someone from this elite swordsmen group saves her. They keep her around, and she is very helpless and CANNOT protect herself. Most of the group members drink something that turns them into a FAKE Oni, the real Oni are not happy and they start a war. Most of the boys die (ssooooooo sad) and the head of the bad people is actually the girls father. They kill him and the lead guy from the elite group proves himself and kills an Oni, while dying himself. In the end only about 2 people from the group survive.