Ayakashi

Adapted from the 2005 adults-only superhuman battle action visual novel game Ayakashi. Ayakashi is a parasite life force that gives superpowers to its host, taking in return the host's entire life force. Kusaka Yuu is a student that lost his will to live after the death of a dear childhood friend, until the day a mysterious girl, named Yoake Eim, appears. From that moment on, his life would never be the same again. Hunted by Ayakashi and their hosts, the power within Yuu awakens and a never ending battle begins.
(Source: Anime News Network)