No Game, No Life: Zero

Izuna and Tet play a game to wager food. As they continue their next game, Tet tells her of a story from 6000 years ago, during the Great War. The Old Deus have driven humankind to the brink of extinction as war is raged upon the heavens and the earth in their quest to become the One True God. Riku, the leader of humanity's last colony along with his sister Corounne, lets another one of his companions die in a Demonia attack while sourcing for information, plaguing him with nightmares and guilt. The colony is at a loss for what to do with their leader in this state, their reducing numbers, and the frequent battles that endanger their lives around the area. Read more on Wikipedia.