Bokurano -10 Character limit is bad

Taken from anidb:

In a summer vacation, a group of 15 children discovered a mysterious man living in a seashore cave, along with his high-tech gadgets. The man claimed to be a game developer creating a video game with 15 giant robots defending Earth from alien invaders and asked those childs to test the game for him. The kids agreed happily at first, but soon they began to find the game horrifying, not only because of its realism but also for the first fatality among them happened right after their first victory. Worse yet, they had no idea how to stop or leave the "game."

First off a small note- Don't watch the anime. The director disagreed with the manga's extremely dark tone and ending, so he went ahead and changed it and made it a semi happy ending. A happy ending in such a dark manga? Screw that >>

Its not that long, but its quite a ride. If you're in a dark mood, give it a shot. Its really not your typical giant robot type of thing. Sure you gain the power to control the infinite power of the robot for a time; But in the end you must pay a price, one of which no one would ever want to pay. Highly recommended

http://www.onemanga.com/Bokurano/