Chaos (カオス, Kaosu?), also known as Chaos, Walker of the Wheel (輪廻王カオス, Rinne Ō Kaosu?, lit. Chaos, King of Reincarnation), is a recurring creature in Final Fantasy series. The main antagonist of the original Final Fantasy, variations on Chaos have reappeared through the franchise as a monstrously powerful demonic figure associated with reincarnation and endless cycles of violence, appearing as either a powerful or final boss, or as a summoned monster.
Appearances
Final Fantasy
Chaos appears as the final boss. A former knight of Cornelia named Garland, when he was defeated in the Chaos Shrine by the Warriors of Light he was sent 2000 years into the past by the Four Fiends. There his hatred would merge with elements of the world, causing the paradoxical creation of the Four Fiends and turning him into Chaos. Chaos would then send the Four Fiends to the future, creating a time loop wherein the Four Fiends would always sent Garland back to the past upon his defeat.
After the Warriors of Light defeated the Four Fiends in the present, who had been draining the elemental powers of the Crystals, the Crystals revealed a portal to the Chaos Shrine of 2000 years past. There they defeated the Four Fiends of the past and re-encountered Garland, who explained the time loop he had created to ensure his survival. Turning into Chaos for the final battle, he would finally be defeated and time loop ended.
Final Fantasy VII
Chaos appears as Vincent Valentine's level 4 Limit, which sees him transform into the creature. Little explanation is given for Vincent's transformations in the original game.
Like all of Vincent's Limits, transforming into Chaos causes the player to lose control of him. Vincent's Defense and Magic Defense are 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 2.5x stronger than Vincent's normal attack, and Satan Slam hits all foes, but with an attack power 5.625x stronger than Vincent's normal attack power, and has a chance to cause Death.
Dirge of Cerberus -Final Fantasy VII-
Chaos returns as a transformation for Vincent, and is revealed to be a pseudo-Weapon, beings created by the planet as a form of self-defense. Chaos was created to work in tandem with the Omega Weapon in the event of a world-ending threat. In such situations, Chaos was intended to destroy all life on the planet that it might be returned to the Lifestream, while Omega would gather up the Lifestream to serve as an ark that life may be reborn on a new planet.
Chaos is bound to a special materia called the Protomateria that would come to be embedded in Vincent's body, allowing him to transform into the creature.
Final Fantasy XI
Chaos appears as the final boss of Final Fantasy XI: The Voracious Resurgence. He is a former Avatar that 5000 years ago waged war on all new life. He was defeated by an alliance of beastmen, but scattered eggs across the land so that he may be reborn.
Final Fantasy XII
Chaos, Walker of the Wheel, appears as an Esper from the world of Ivalice. The Esper of Wind, he represents the zodiac sign of Taurus, the Bull. According to myth, when sent to the world of man by the gods, he became lost, dying, and being reborn countless times before rebelling against the gods in anger for his fate. Chaos can be encountered in the Necrohol of Nabudis, where he was sealed following the Battle of Nabudis.
Final Fantasy XII: Revenant Wings
Chaos returns as a Rank III flying-type Fire summon. He is Mydia's final summon and appears in "Mission 39: Wayward Soul" at the Feol Warren. Chaos is backed up by many other Fire-type Yarhi, including Wyverns.
Final Fantasy XIV
Chaos appears as the boss of 
Alphascape V1.0 and its Savage version. He is a simulacrum created by Omega for the purposes of a tournament to find the strongest lifeform to pit itself against. Owing to the robotic Omega's literal lack of imagination, it pulled most of its test subjects from history, literature, and myth. Chaos is said to be an ancient evil defeated by the first heroes, though the tale is so ancient that Cid Garlond is unable to determine if it is history, myth, or based on another reality altogether. Ultimately, Chaos emerges as the strongest lifeform in its bracket and is pitted against the Warrior of Light in the final match of the tournament.
Final Fantasy Tactics A2: Grimoire of the Rift
The Ivalician Chaos returns as Scion and can be summoned by anyone who has the Ring of the Wheel accessory equipped. When summoned, he uses his Tornado attack, inflicting Wind-based damage on all enemies.
Final Fantasy Legend III
Chaos appears as a minor antagonist and servant of the Water Entity. He is fought as a boss in the Castle of Chaos.
Final Fantasy: The 4 Heroes of Light
Chaos appears as the main antagonist and final boss. Having appeared 300 years ago when humans nurtured darkness in their hearts, he was sealed away by the dragons of Spelvia, but broke free 15 years prior to the events of the game. When the king of Spelvia Rolan tried to stop him he had his servant Lucifer corrupt him. When the heroes of the present day defeat the corrupted Rolan, they learn of Chaos' existence and travel 15 years to the past to defeat him and his servants.
Final Fantasy Dimensions II
Chaos appeared as a boss in the free-to-play version during the 原初幻想のカオス (Gensho Gensō no Kaosu?, lit. Chaos of Primordial Fantasy) event.
Dissidia Final Fantasy series
Chaos appears as the primary antagonist and final boss of Dissidia Final Fantasy and Dissidia 012 Final Fantasy. He is the god of discord that opposes Cosmos, the goddess of harmony, both being equal in power. During Dissidia Final Fantasy, Chaos and his forces tip the balance of power in their favor, setting the stage for their victory. Dissidia 012 Final Fantasy explores this shift in power in-depth, and the actions taken by hero and villain alike that set the stage for the thirteenth cycle.
Theatrhythm Final Fantasy series
Chaos from Dissidia appears as the final boss Theatrhythm Final Fantasy. He returns as a boss and playable character in Theatrhythm Final Fantasy Curtain Call and Theatrhythm Final Fantasy All-Star Carnival.
Pictlogica Final Fantasy
The Final Fantasy, Final Fantasy XII, and Dissidia incarnations of Chaos appear as bosses.
Final Fantasy Airborne Brigade
Chaos from Dissidia appears as a boss.
Final Fantasy Artniks
Chaos from Dissidia appears as a card.
Final Fantasy All the Bravest
Chaos from Final Fantasy appears as a boss.
Final Fantasy Record Keeper
The Final Fantasy and Mobius incarnations of Chaos appear as bosses.
Final Fantasy Brave Exvius
The Final Fantasy incarnation of Chaos appears as the featured boss of the Series Boss Battle: Chaos. It uses a vast arsenal of elemental attacks, status ailments and powerful debuffs.
An invading force known as Chaos act as the main antagonists of Season Five: Sword of a Decision. They invade Lapis offering a proposal of surrender: Letting them rule the world in exchange of granting land to humanity. The invasion and the proposal causes a schism within the main characters, as their allegiances are split.
Chaos is capable of conjuring a mysterious dark red rain from which strange monsters known as Chaoskin emerge. Losing consciousness as a result of battle against these monsters, causes the victim to be encased in a crimson crystal.
Mobius Final Fantasy
Chaos appears as a main antagonist of season 1. Once a the tyrannical lord of a village, Chaos was killed in a duel with the ancient hero Cid. After falling into the ocean, Chaos lived on through his resentment of Cid, and his tainted soul began to create monsters out of water, collectively dubbed "Chaos" after him. Chaos became an inseparable part of Palamecia, existing as long as the world still does and returning in due time after every defeat.
Chaos from Final Fantasy also appears as a card, while Chaos, Walker of the Wheel, from Final Fantasy XII appears as a Sicarius with his own obtainable card.
Stranger of Paradise Final Fantasy Origin
Chaos serves as an important plot device in Stranger of Paradise Final Fantasy Origin, an alternate universe prequel to the first Final Fantasy game. Jack Garland and his party begin with the singular goal of killing Chaos, initially believing Chaos to be a corporeal being. As Jack gains more of his lost memories, he discovers that "Chaos" is a phenomenon that stems from the darkness of humanity and that the Lufenians have goaded Jack and his party to act in their interest of using Cornelia as their experimental dumping ground for the darkness. The burgeoning darkness in Jack's soul leads him to the Darkness Manifest, a demonic being that resembles the Chaos entity from the original Final Fantasy.
Disgusted by the Lufenians' manipulation, Jack severs his connection to the Lufenians, and battles the Darkness Manifest to fully-absorb its powers to become Chaos himself. Jack collaborates with his friends to utilize his newfound dark powers to train the Warriors of Light and wrest control of Cornelia from the Lufenians.
Final Fantasy Trading Card Game
Chaos from Final Fantasy, Final Fantasy XII, Dissidia, and Theatrhythm appear as cards.
Non-Final Fantasy guest appearances
Ehrgeiz: God Bless the Ring
Chaos from Final Fantasy VII can appear as an afterimage when Vincent's gun is charged. Each charged shot that is fired can create an afterimage of his Limit alter egos upon a successful hit.
Puzzle & Dragons
Chaos appeared as part of the Final Fantasy collaboration that ran from November 2 to 15, 2015. He was fought as a boss, and dropped Cloud's Buster Sword upon defeat. He was also obtainable as a 4-star unit, with a devil type and dark and light elements.
Etymology
Chaos refers to the void state preceding the creation of the universe in Greek creation myths, or to the initial "gap" created by the original separation of heaven and earth. Chaos also refers to a state of disorder, disharmony, dissonance, lawlessness, and unpredictability.