Mercy (マーシー) by Shiki Kouyuu
Genre: Seinen, Action, Fantasy, Supernatural, Horror, Award-winning
Synopsis: The Immortals arrived 500 years ago. They were originally celebrated, even worshipped, but as they began to use humanity for their own ends they became reviled. Now, an immortal girl hunts for her own kind in their deep lairs to bring about the end of their reign of terror.