Gantz

Alternative Name(s):	ガンツ 杀戮都市
Year of Released:	2000
Status:	Ongoing
Author(s):	OKU Hiroya
Artist(s):	OKU Hiroya
Genre(s):	action, ecchi, mature, romance, sci-fi, seinen, supernatural
Rating:	Average 4.70 / 5 out of 81 total votes. 4.5
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Summary

Upon dying, Kurono Kei and selected others are forced to fight aliens at the beck and call of a black sphere named Gantz. Using super high tech weaponry and uniforms, their strength, agility, and abilities increase immensely.

From Anime News Network
Somewhere in Tokyo, there is a room. In that room is a black sphere. Periodically, people who should otherwise have died are transferred to the room. There, the sphere gives them special suits and weapons, and sends them out on a mission to kill aliens here on Earth. While these missions take place, the rest of the world is largely oblivious to them. These missions are lethal - few participants survive them. The sphere calls the shots, and it's not the slightest bit nice. It's name... Gantz.