Gantz

GantZ is Japanese comic book and animated series, written by Hiroya Oku.
All characters of the stories have just died and woke up in some strange place and received an order - to kill a pointed person (referred as alien) or to die trying. What for? Why me? What exactly is happening? - nobody knows.
Story characters are not superheroes, but ordinary people confronted with a situation which is completely beyond their understanding and capabilities. The world they just have left was cruel, brutal, and lacked of justice, but now they found themselves to be in much bigger troubles they could have ever possibly imagined.
And oh, I don't reckon that the author have ever heard about something called 'happy ending'.