Gantz

If you are chosen by the bizarre black sphere known as Gantz, you are already dead—yet you might be able to reclaim your mortality. First, Gantz demands that you undertake brutal missions of madness, killing aliens hidden among the population. It is your only chance and you have no choice. You must play this disturbing game. And if you die again—and you likely will—it's permanent.

(Source: Funimation)