Chaos (Final Fantasy XIII)

Chaos (カオス) is a matter in the Fabula Nova Crystallis mythos which comes from Valhalla. It is the blessing Etro granted to mankind. However, chaos is dangerous in the fact that if unbound would turn the world into a timeless wasteland.

Datalog
''The chaos of the Unseen Realm is a malevolent energy that seep through into the world of mortals. The existence-defying darkness witnessed by Lightning is an immense and inexorable force, a creeping doom that she is not even certain acts with a unified purpose or will.''

''Where the chaos of Valhalla leaks into the mortal realm, the laws of the physical world are undone. This paradoxical energy seeks to return all existence to the Unseen Realm, but at times it seems to show an almost affection for those who share an affinity with chaos.''

Background
A side effect of Bhunivelze creating the fal'Cie in the Visible World, chaos manifested in the Invisible World by the time Etro entered the realm and met Mwynn in her final moments. Taking Mwynn's place in protecting the world balance, Etro placed a piece of chaos inside each human. This act, giving humanity a heart, restored the world balance while Etro held the rest of chaos under her control within Valhalla.

Story
Due to Etro's intervention in changing the fate of Lightning and other Cocoon and Pulse l'Cie, chaos began to seep out of the gate, but Etro reversed time and sealed the Chaos back in Valhalla. Lightning, who had been consumed by the chaos, was dragged along the Historia Crux and ended up in Valhalla.

Caius Ballad possesses the "Heart of Chaos", said to be Etro's own heart, as a result of the goddess resurrecting him after he sacrificed his life to use an Incarnate Summoning (essentially transforming into Bahamut). This has granted him immortality along with the apparent ability to Incarnate Summon at will, becoming Chaos Bahamut. It also grants him some affinity or control over the chaos of Valhalla, which may have allowed him to create the paradox effect.

Caius's goal is to kill Etro in order to allow chaos to consume all of time, turning the world into Valhalla. Though he is defeated, this results in Etro's death as well. The game ends in a cliffhanger as chaos apparently consumes the world.