The Game of Souls is the name of the overall world in which a multitude of stories take place. It is a game of order versus chaos in which the characters aren't aware they are playing. You, me, and the characters involved have no clue that it's just some game played by the Umbra.
The Umbra is made up of all the energy of the world and it's divided by order and chaos. Each side vying for the most energy. Order plays with only a few tools at his disposal - relying on free will to win. While Chaos interferes with life in every fashion it can possibly imagine. And this is that story.