Gantz

Gantz tells the story of a teenager named Kei Kurono who dies in a train accident and becomes part of a semi-posthumous "game" in which he and several other recently deceased people are forced to hunt down and kill aliens. The missions in which they embark upon are often dangerous. Many die--again--on each mission, but they are replaced by others in the same manner as Kei Kurono's appearance. [from en.wikipedia.com]