- "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."
- —Caius explains the role of the Heart to Noel in a Dying World.
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.
- See also: Fragment/Oracle of Etro.
- 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.
- 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.
During a time before the War of Transgression, Etro recognized the l'Cie and Guardian Caius. She saw his dedication to protect the seeress Yeul and took pity on him. Etro released Caius from his fate as a l'Cie, and "blessed" him with the Heart of Chaos, thus granting him immortality, and enabling him to protect Yeul's reincarnations throughout time. Etro's act, however, would be seen as a curse of eternity to Caius, dooming him to bear the sight of hundreds of Yeuls dying before his eyes. 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 Noel Kreiss, a young hunter born in a Dying World around the year 700 AF. Caius 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 winning causing him to resolve to kill Etro himself in Valhalla. Caius does not succeed in this as the goddess is protected by her champion, Lightning.
As Lightning's sister, Serah Farron, and Noel, begin their journey through time to find Lightning, Caius creates a number of paradoxes throughout the timeline in hopes of achieving his goal while placing the two in danger from both his past self and the Proto fal'Cie Adam, both seeing them as a threat to the timeline. When Serah and Noel defeat Caius in Valhalla, he tries to provoke Noel into killing him. Although Noel refuses, Caius succeeds in destroying the heart when he forces Noel's blade through it as Noel still holds it in his hands. As such, Noel did not kill Caius, and the blessing of the goddess did not pass unto Noel. This act not only killed Caius but also killed Etro, and with Etro's death, the chaos of the Unseen World was unleashed upon Bhunivelze and Gran Pulse, causing the world to become like Valhalla.
If the player defeats Caius in Noel's dream world with Paradox Scope active, they trigger the paradox ending, "Heir to Chaos," where Noel kills Caius and inherits the Heart of Chaos. Doing so makes Noel immortal and he inherits Caius's memories. Noel, as well as the fragment of that paradox ending states, says that he will not be like Caius and he will save him, Yeul, and Serah.
"Heart of Chaos" (混沌の心臓, Konton no Shinzō?), which was composed by Naoshi Mizuta, plays during the final battle of Serah and Noel against Caius in Valhalla. It is a fast-paced arrangement of "Caius's Theme".
- The Heart of Chaos is one of the many story elements of Final Fantasy XIII-2 referenced in the song "Worlds Collide," one of the game's battle themes.