[DahGame180]

I've had a anime that I read long ago but can't quite remember the title of the anime. If you could please help me out I would be very Grateful.

Here are some details that I remember about the anime...

-Two (Kids) are the main characters.
-One of the kids family is a family of assassins.
-They get lost in some sort of game.
-The two kids later round up some members to be part of there group.
-There's some sort of gang having to do with Spiders.
-One of the main characters father is trapped in the game.
-They train in the game with some girl that looks young buts really old, to become super strong.
-I Remember they trained in this game and they had to fight to survive to go against other people in this game.
-The kid that comes from a family of assassins somewhere in the series pulls a guys heart out in a matter of seconds, he also is able to walk with the sound of his footsteps.

Thanks to anybody who can help me out!!