Marionette ~Ito Tsukai~

The main character is Hiroki Mikage, apparently nothing more than a gifted but unremarkable high school student, although this facade hides an almost physical Darkness, a profound sense of restlessness and dissatisfaction menacing his own sanity.

However this is merely the prelude to even darker events, when Hiroki, while going to school, notices a strange violet light coming from the riverbank beside the road and finds a mysterious violet gem(?) scintillating with an eerie inner light...

[Edited from a forum post by Baldo]