From the introduction:
Welcome to the Keeper Games! If you're here, you killed an active Keeper, and as a result, destroyed their entire realm and everyone in it!
Now it's your turn to try your hand at being a cosmic force of creation! Beware though, once your realm reaches a certain point of development, the force behind this system will start pitting you and your realm against one of 9998 other Keepers and their realms, with one side being designated as invader, while the other is set as defender.