- "Soul wrought of terra corrupt, quelling impurity, purging the stream to beckon forth an ultimate fate. Behold mighty Chaos, Omega's squire to the lofty heavens."
- —Lucrecia Crescent
Chaos appears in Final Fantasy VII and its sequel Dirge of Cerberus -Final Fantasy VII-. Chaos has a minor role in Final Fantasy VII as it appears as just a Limit Break for Vincent Valentine, however, it has a much larger role in Dirge of Cerberus -Final Fantasy VII-, having a backstory, and being the cause of the major conflicts in the game.
Vincent as Chaos is the only incarnation of Chaos in the entire Final Fantasy series that is portrayed as clearly on the heroes' side, though not necessarily benevolent, but only because Vincent himself is.
The role Chaos has in Final Fantasy VII is as a Limit Break for Vincent. He is Vincent's ultimate Limit Break and can be obtained by visiting Lucrecia's Cave, watching the events there, returning after fighting ten battles and examining the altar to receive the manual, along with the Death Penalty weapon which is Vincent's ultimate weapon. To use the manual, Vincent must have already obtained his other Limit Breaks.
Besides being a Limit Break, Chaos has no other purpose in the game, and if Vincent is not obtained as a party member, as he is an optional character, the player can bypass seeing Chaos at all.
Like all of Vincent's Limit Breaks, the player has no control over Chaos once Vincent transforms into him, his stats change and he uses unique attacks. Vincent's Defense and Magic Defense will be doubled while he is Chaos and he uses the attacks Chaos Saber and Satan Slam. Chaos Saber hits all foes with an attack power 2.5 times stronger than Vincent's normal attack and Satan Slam also hits all foes, but with an attack power 5.625 times stronger than Vincent's normal attack power, and also has a chance to cause Instant Death.
In the PC version re-release of Final Fantasy VII, obtaining Chaos unlocks the Chaos achievement.
The nature of Chaos is expanded upon in Dirge of Cerberus, where it is revealed to be a semi-Weapon that acts according to the will of another Weapon, Omega. The legend of Chaos and Omega's existence as Weapons that the Planet calls forth in a time of threat were only recently discovered by Dr. Lucrecia Crescent and Dr. Grimoire Valentine.
Chaos is a being born of negative and dark memories and the sins within a naturally reoccurring stagnated and tainted form of Lifestream, its purpose being an executioner and harbinger of death who comes during a period of great catastrophe and mass planetary-scale destruction, allowing the rest of the Planet's Lifestream to be gathered to a new planet to continue the cycle of life.
Although Vincent still retains the ability to turn into Chaos and maintain control of himself at the start of Dirge of Cerberus, the player is unable make him turn into this form during gameplay like they can with his Galian Beast form. Chaos becomes playable in Dirge of Cerberus during the game's final level. The form allows him to hover briefly, perform a large combo consisting of various claw swipes and kicks ending with a sphere of energy, perform a large jump equal to a double jump, and wield a more threatening variation of the Death Penalty gun (his final weapon from Final Fantasy VII), which gains power as it collects the souls of the wicked.
Stated in the Omega Reports, Chaos was first described in an ancient Cetra tablet. Later, a team of scientists that included Grimoire Valentine and Lucrecia Crescent found a small pool of tainted Lifestream within the Crystal Cave, hypothesizing that Chaos would arrive here at the destined time, and later excavated the Protomateria from its depths.
With samples of the Mako and the newly discovered Protomateria brought back for experimentation, Grimoire died in an experiment associated with the discovery. Shinra used the tainted Mako in an attempt to create stronger SOLDIERs within Deepground, which resulted in the birth of Nero the Sable. The scientific investigation of Chaos and Omega later comes to a complete stop when other scientists dismiss and criticize Lucrecia's report.
Chaos's nature is revealed explicitly later in Dirge of Cerberus -Final Fantasy VII-, such as what exactly he is, how he was discovered and how he links to Vincent. Chaos makes frequent appearances throughout the game and most of the information about him is revealed by Shalua Rui, Shelke, and Lucrecia.
Chaos is first seen in Dirge of Cerberus at the end of Chapter 3 when Vincent Valentine meets Rosso the Crimson. The two fight during a cutscene with Vincent ultimately losing, as he is unaware of the strength of his opponent, and being hurled through a building wall. When he emerges a few seconds later he is Chaos and uses his power to knock Rosso away. As in Final Fantasy VII, he still seems to have control over himself while in this form, as the Protomateria is still within his body.
Vincent awakes later to find himself in a regenerative chamber in the World Regenesis Organization Headquarters. Shalua, who had found Vincent and brought him to the WRO to recover, releases him and tells him about Lucrecia's research on Chaos. Vincent does not seem to know that Lucrecia was researching Chaos and Omega, but before he can ask more about Chaos, Deepground attacks, forcing him to leave his questions until later and fight.
Chaos is not mentioned again until Chapter 5 when Vincent is in the Nibelheim mansion. When he finds a piece of Materia that contains a recording of Lucrecia's, Vincent hears Lucrecia explain to him what Chaos and Omega mean to the world. Lucrecia tells Vincent in the recording that Omega will be the end of the world, though she does not mention exactly what part Chaos plays in this.
After battling his way through the mansion, Vincent comes across Rosso once more. After a small talk, Rosso flees and Vincent chases after her to the mansion entrance where he fights a Black Widow robot. After the fight, Rosso catches Vincent off-guard and removes a piece of Materia from his chest. As she does this, Vincent, who has collapsed to the ground, transforms once again into Chaos, but instantly transforms back. Rosso comments it appears that Vincent cannot control Chaos without the Materia she just extracted, but before she is about to kill him Yuffie arrives and rescues him.
From this point on, Chaos becomes more and more unstable within Vincent. Aboard the Shera during Chapter 7, Vincent briefly becomes Chaos, slashing out at nothing, tearing deep gouges into the wall. He is only brought out because he sees an image of Lucrecia walking toward him, snapping Vincent's mind back into place.
He gets another bout of Chaos outside the Shinra Headquarters in the ruins of Midgar at the end of Chapter 8. It is here that Shelke explains to him that, after the Protomateria was taken from him, his mental defense against Chaos was weakened, and he was slowly losing all control. She also explains that Chaos, while a part of Vincent, acts independently from Vincent himself and explains that Chaos shall be born before the Planet dies and shall gather together all life for Omega.
Chaos appears once again after Azul knocks Vincent unconscious after their fight at the end of Chapter 9. Chaos goes wild, impales Azul with his own gun and then knocks him off the elevator they were on. Vincent has a flashback of his time after he was shot by Hojo and Lucrecia was trying to revive him. Hojo comments on Chaos and Omega, but no information is given on them.
Chaos is next mentioned in another flashback in Chapter 10. Lucrecia and Grimoire Valentine, Vincent's father and a fellow scientist interested in Chaos and Omega, are in the Crystal Cave which, according to Grimoire, is where Chaos is supposed to awaken.
Hojo, or rather, a digital version of him inside Weiss, explains to Vincent near the end of Chapter 11 exactly how Chaos exists within him. Due to Hojo's experiments of Vincent many years ago, Vincent's body became more durable and thus Chaos could exist inside him. He also says that Chaos is born of a tainted Lifestream.
Vincent tries to use Chaos to fight against Weiss but fails. Vincent hallucinates that Lucrecia is standing in front of him; she tells him to try and control Chaos, not let Chaos control him. Vincent does so successfully by utilizing the Protomateria within Weiss and uses Chaos's power while retaining his human form and defeats Weiss.
At the end of Chapter 11, Chaos finally takes full control over Vincent's body when Omega is summoned to, according to Shelke, counterbalance Omega's appearance. Shelke then uses a SND to retrieve the Protomateria and return it to Vincent so that he can control Chaos once more.
Before Vincent gains complete control over Chaos the player learns, through another flashback scene, what Lucrecia did to Vincent so that Chaos resides in him. It seems that after Vincent had been shot by Hojo and experimented on, he was still dying. Lucrecia tried to save him by infusing a tainted Lifestream into him as she believed that her thesis on Chaos could potentially save him. However, while this stopped Vincent's body from decaying, it also meant he was permanently Chaos with no control over it. It was then that Lucrecia discovered the Protomateria and used it so that Vincent could control Chaos, however, although it is not said, it appears that Vincent resumed decaying as when Lucrecia tries to use it, her computer screen flashes red, like a sign of failure.
It is unknown exactly what happens to Vincent after this or how he stops decaying, however, due to the events of Final Fantasy VII and flashbacks that are seen, it can be assumed that Vincent woke up and realized what he had become and then went into his deep slumber.
Vincent as Chaos goes to combat Omega and Weiss. After the battle with Weiss, Omega begins ascending so Vincent uses Chaos to destroy Omega as they are opposites. However, it appears that after this Vincent, along with Chaos, dies. During the ending movie after the credits, we see Vincent, who did survive fighting Omega, saying to Lucrecia that Chaos and Omega returned to the Planet.