Gantz.

About Gantz

Gantz is a Japanese manga series written and illustrated by Hiroya Oku. It was serialized in Shueisha's seinen manga magazine Weekly Young Jump from June 2000 to June 2013, with its chapters collected in 37 tankōbon volumes. It tells the story of Kei Kurono and Masaru Kato, both of whom died in a train accident and become part of a semi-posthumous "game" in which they and several other recently deceased people are forced to hunt down and kill aliens armed with a handful of futuristic items, equipment, and weaponry.