Ayakashi

Ayakashi (アヤカシ) is a 2005 visual novel developed by Crossnet. An eroge sequel was later released in 2006 with the name Ayakashi H. A twelve-episode anime adaptation was aired in 2007. A bundle release of the game called Ayakashi Pack was released on December 14, 2007 to celebrate the release of the anime, containing both Ayakashi and Ayakashi H.

Plot
Ayakashi is a parasite life force that gives superpowers to its host, taking in return the host's entire life force. Yū Kusaka is a student that lost his will to live after the death of a dear childhood friend, until the day a mysterious girl, named Eim Yoake, appears. From that moment on, his life would never be the same again. Hunted by Ayakashi and their hosts, the power within Yū awakens and a never ending battle begins.