Isn't this exactly like No Game No Life? The strong being such as Gods perished while the weaker beings like the Imanity(Humanity) managed to survive even though they were one of those races who got caught in the crossfire of the war. After that, a one true god emerged and created a set of rules. (These rules basically crippled the stronger beings. They had to use their wits more in order to win and survive.)
The rules were absolute and everybody, even the Gods, have to follow it. Everything was decided by a game and the participants of the said game has to bet something of equal value. It was definitely a really great novel that I enjoyed reading.
Here are the 10 Pledges:
1) All murder, war, and robbery is forbidden in this world.
2) All conflict in this world will be resolved through games.
3) In games, each player will bet something that they agree is of equal value.
4) As long as it doesn't violate pledge three, anything may be bet, and any game may be played.
5) The challenged party has the right to decide the rules of the game.
6) Any bets made in accordance with the pledges must be upheld.
7) Conflicts between groups will be conducted by designated representatives with absolute authority.
8) Being caught cheating during a game is grounds for an instant loss.
9) In the name of god, the previous rules may never be changed.
10)Let's all have fun and play together!
Oh, and, by the way, the God of Games was the one who won the war and obtained the 'right' to become the One True God. After his ascension, he immediately set this 10 Pledges as a kind of rule that everyone should follow. All the conflicts are decided by games. There's also the fact that every races were given a single chess piece as the kind of thing that represents the fate of their whole race. That's why it is called as the Race Piece. If they were to bet that race piece and lose, the fate of their whole race will be under the mercy of the winner.
BTW, Imanity got the King piece. The most useless and most important piece.