Gantz is a really dark anime which gives you a thought of what happens when you die.
The Main Character of this story is Kurono Kei, A Selfish High school student with no feelings for anyone but himself. One day his life ends quickly when he is forced to help a homeless man off the rails in a Subway station with the help of his School friend Katou Masaru.
Both of them are killed in the impact with the train. Then after that, both of them are taken to this room were a couple of other people have died as well, whilst there they come across this strange black ball called Gantz which somewhat controls the fate of everyone's lifes. They are told by Gantz to hunt down these aliens who are somewhat invading are planet-Reasons unknown.
Gantz is a really powerful series which you would need to take in, It gets even more dark along the way so Be aware of what you see.
So Go watch this Show and prepare for the Ultimate Ride of your life!