The Heart of Chaos beats in my chest—a manifestation of Etro. Should this heart stop beating, the goddess will die once again. When the goddess dies, the chaos of Valhalla is unleashed. The chaos has the power to warp the timeline and destroy the past.
The Heart of Chaos (混沌の心臓, Konton no Shinzō?) is a manifestation of the goddess Etro, and it is her own heart. It resides within the chest of Caius Ballad in Final Fantasy XIII-2. The "Heart of Etro" Caius possesses is Etro's "physical" heart, and not a kind of heart humans have that defines their person. Etro's heart uses different Japanese term than the "personality heart". Etro's soul exists outside of it, but it's not specified where. The English localization makes no distinction between people's hearts and souls, but the two are separate entities in the Japanese version.
The Heart of Chaos is the goddess Etro's own heart, given to Caius. It is the font of the immortal Guardian's power. Only if another potential Guardian defeats him and takes this power can Caius truly be killed. And the only person remaining in this world with that potential is Noel.
If the new Guardian has enough strength of will, he can release the Heart's power and put an end to immortal destiny. If he wavers, however, he will be enslaved in chains of chaos, and become the latest in a line of eternal servants.
- Words of Protectors Past
The Heart of Chaos does not grant the Guardian power—it eats away at their physical self. Caius did not gain an immortal body, but was cursed with eternal life.
Heed my words, Noel. You are the last protector, and the only one who can wrest the Heart's strength from Caius. In the instant you strike the life from his body, you shall either inherit the curse, or break its eternal grip. Only your will can slay fate one way or the other.
- —Ultima Brand
- Words of Protectors Past
A massive battle once raged on Gran Pulse. Paddra was besieged by a great army that sought the life of the seeress. In the face of this assault, Caius chose to perform an 'incarnate summoning.' This ritual merged his essence with that of the Eidolon Bahamut, and drew forth all their latent power.
An incarnate summoning expends the life force of the summoner, and thus Caius did perish. But he returned from death's domain, an immortal heart beating within his breast. That dark summons is now his to wield at will—as long as the Heart of Chaos pumps eternity through his veins.
- —Chaos Brand
- Yeul's Confessions
Etro granted the most selfless of gifts. Her power and strength flowed through the gate to the Unseen World, and with each passing second she grew weaker and weaker.
Now the goddess sleeps within the Heart of Chaos. Should she cease to exist, then the power that she granted to humanity, the power that makes their universe whole, will disappear. When the sands of time run out on the goddess, so too do they on the world of man.
- —Etro's Sorrow
During a time before the War of Transgression, Etro recognized the l'Cie and Guardian Caius for dedication to protect the seeress Paddra Nsu-Yeul. In what she considered an act of mercy, Etro released Caius from his fate as a l'Cie and "blessed" him with the Heart of Chaos so that he can protect Yeul's reincarnations throughout time. Etro's act would be seen as a curse of eternity to Caius, as his body suffers pain despite being considered immortal, and he is doomed to watch hundreds of Yeuls die. Driven mad, Caius waits centuries for the right person able to pierce the Heart of Chaos, therefore killing Etro and freeing him from his curse.
The only person capable of freeing Caius is another would-be Guardian who can either destroy the heart or inherit it. Caius finds his pawn in Noel Kreiss in the year 700 AF and takes Noel under his wing and trains him to become a Guardian to Yeul like himself. When Noel shows the right potential, Caius challenges the unwilling boy to battle, hoping that Noel will release him. The duel ends with Caius's victory, and he resolves to kill Etro himself in Valhalla, the realm dislodged from time where Etro is said to reside. Caius does not succeed as the goddess is protected by her champion, Lightning.
As Noel and Lightning's sister Serah begin their journey through time to find Lightning, Caius creates a number of paradoxes throughout the timeline. Caius places the two in danger from both his past self and the Proto fal'Cie Adam. When Serah and Noel defeat Caius in Valhalla, he tries to provoke Noel into killing him. Although Noel refuses, Caius forces Noel's blade through the heart in what he calls an eternal paradox. This kills Etro as he had planned, causing Gran Pulse and Valhalla to merge into Nova Chrysalia as the chaos Etro had kept at bay is released. Caius had failed to realize he has doomed himself to another form of immortality, as he has been been bound to the chaos itself by Yeul's love for him—as revealed in Lightning Returns: Final Fantasy XIII, as long as Yeul doesn't want to let him go, Caius cannot disappear.
If the Noel defeats Caius in Noel's dream world with Paradox Scope active, the paradox ending "Heir to Chaos" happens: Noel kills Caius and inherits the Heart of Chaos. Noel becomes immortal and he inherits Caius's memories, but states that he will not be like Caius, and will strive to save him, Yeul, and Serah.
- The Heart of Chaos is one of the many story elements of Final Fantasy XIII-2 referred to in the song "Worlds Collide," one of the game's battle themes.
- The concept of Etro placing a heart inside a human was first glimpsed in the Fabula Nova Crystallis: Final Fantasy mythology video Square displayed in 2011, and seems to be the inspiration for Caius's Heart of Chaos.