After death, humans go to either the eternal void or are reincarnated. However, where there's doubt as to were to place some occasional souls, these instead arrive at bar Quindecim, with no memories of having died. Decim, the bartender, challenges them to the Death Game.
Wherein they wager their "lives" and reveal their true natures. Decim himself is actually an arbiter in charge of deciding the fate of those that arrive at Quindecim - by pulling the darkness from deepest within them, he judges their ultimate fate.