Zero: Black Blood.

Rei Suzumura, Zero the Silver Fanged Knight, is patrolling on his own when he encounters the white Horror Ring, who has created a commune where Horrors and humans live together in harmony, so long as one human a month allows themselves to be devoured by the Horrors. Rei, along with Makai Priest Cain and the knight-in-training Yuna are ordered by the Makai Senate to slay Ring and free the humans under his thrall by means of Yuna's mother the Makai Priestess Iyu.