Darker than Black

Also known as:  Darker than Black: Kuro no Keiyakusha, Darker than Black: The Black Contractor, DTB

The story of Darker than Black: Kuro no Keiyakusha begins ten years ago in Tokyo, with the appearance of a bizarre field that is impossible to analyze. Coinciding with this "Hell's Gate" is the appearance of humans possessing new forms and new powers. These criminally insane beings are fearfully called "Contractors" and sometimes brutally murder others.

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Producers, official website and other details: AniDB, ANN, Wiki