Gantz - The game Japan

Gantz is a Japanese manga series written and illustrated by Hiroya Oku. Gantz tells the story of Kei Kurono and his friend Masaru Kato who die 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. Both the manga and anime are noted for their heavy violence and sexual content.