Gantz

Gantz is a Japanese manga series written and illustrated by Hiroya Oku. Gantz tells the story of a teenager named Kei Kurono, among others, who die but find themselves brought back in perfectly healthy bodies and forced to participate in a "game" where they must hunt down and kill aliens. Those who die on missions are quickly replaced by others in following missions. The series is known for its graphic violence, sex, action and similar mature themes.