Chaos is the final boss of Unlimited Saga. Chaos is actually a powerful form of energy, which the Arcanians (an ancient civilization) tried to experiment on. They created the Seven Wonders in order to harness its power, but the power ended causing destructive weather phenomena all around the world, as well as creating many creatures, such as monsters and even the humans. The Creator, worried that Chaos' power would consume the world, extinguished the Arcanians; but horrified at what he's done, decided to leave the world.
Chaos will manifest after the heroes conquer the Wonders, warping itself and the character to the moon's surface for the final battle.