Chaos is some form of demi-god and is the original guardian of the magical Chaos Emeralds. Long ago, it protected the Emerald Shrine before the land it was on ascended into the sky as Angel Island. Creatures called Chao lived around the shrine and played together peacefully with Chaos as their caretaker. However, this peace was disrupted by the ancient Echidna people, who were savagely conquering surrounding countries. When they discovered the Chaos Emeralds, they intended to steal them and gain their power, but Tikal - daughter of the tribe's chief Pochacamac - had befriended the Chao and tried to convince her father to leave the Emerald Shrine alone. Pochacamac would not listen and gave the order to charge, and so the Echidna warriors shoved Tikal aside and slaughtered the defenseless Chao as they headed for the Master Emerald. Chaos then appeared before Pochacamac and his warriors, enraged by their barbaric actions, and slaughtered them all.
Chaos became consumed by rage, calling upon the negative energies of the Chaos Emeralds to lay waste to the world. Tikal, desperate to calm the monster, prayed to the Master Emerald and used the Chaos Emeralds to bind her soul to Chaos and seal the both of them within the Master Emerald. Following Chaos' imprisonment, the few survivors of the Echidna race left behind a handful of relics warning of a "God of Destruction", responsible for destroying their civilisation.
In modern times, the relics and stone tablets telling of Chaos were discovered by the evil scientist Doctor Ivo Robotnik, aka Dr. Eggman. The doctor had moved his base of operations to the region known as the Mystic Ruins, which was the city of the Knuckles Clan in the ancient past, and there the doctor learned of the "God of Destruction" which would consume the seven Chaos Emeralds and destroy the world. Dr. Eggman believed that if he acquired the emeralds and fed them to Chaos, he would be able to control the creature and use it to destroy the city of Station Square, upon the ruins of which he would build Eggmanland, capital of the globe-spanning Eggman Empire.
Eggman learned that Chaos was sealed inside the Master Emerald and so flew his Egg Carrier battleship to the floating Angel Island. The Egg Carrier fired on the Master Emerald, shattering it into pieces and releasing the enraged Chaos, as well as the spirit of Tikal. With Chaos released, Dr. Eggman went in search of the seven Chaos Emeralds, which were each in different locations.
The first emerald was found in the possession of Eggman's arch nemesis Sonic the Hedgehog, whom the doctor challenged for the gem. Though Eggman lost the fight, he still won the prize by snatching it out of Tails' hands. Eggman fed the emerald to Chaos, which transformed once the gem entered its body. A desperate race began between Sonic and Eggman to find the remaining Chaos Emeralds before the other, but Eggman would keep interfering with Sonic's efforts and stealing the emeralds from him.
The second emerald was found in Casinopolis in Station Square, but Eggman ambushed Sonic and Tails as they left the casino and took it. Eggman would acquire two more emeralds after Sonic dropped them following a fight with Knuckles the Echidna. Two more emeralds were in the possession of different parties: one was Amy Rose and a small bird she had taken into her care, the other had been swallowed by a frog. One emerald remained unaccounted for, but all of the emeralds were seemingly lost after Sonic and Knuckles fought and defeated Chaos aboard the Egg Carrier.
Although Chaos had apparently been defeated and Dr. Eggman's scheme had been thwarted, the monster was still alive and headed for Angel Island, where it attacked Knuckles and took the six Chaos Emeralds that Knuckles had brought home with him. The final emerald had been found by Tails and was installed aboard his Tornado 2 plane, but the plane had been stolen and subsequently crashed by Big the Cat following the fall of the Egg Carrier. Sonic and Tails found the plane in the jungle, but Chaos appeared and grabbed the last emerald before the heroes could reach it.
Now that it had achieved its perfect state with all of the Chaos Emeralds, Chaos attacked Station Square, flooding the whole city with its watery body. Dr. Eggman attempted to destroy the monster with a second Egg Carrier, but that ship was immediately destroyed by a blast of energy from Chaos' mouth. Just then, the spirit of Tikal appeared before Sonic. Tikal had been helping Sonic throughout his adventure and had shown him glimpses of the past and what had happened to her and Chaos. She suggested that Chaos be resealed within the Master Emerald, but Sonic felt that wouldn't be of help to Chaos, since the creature would still be driven by anger and pain.
After absorbing their power, Chaos discarded the Chaos Emeralds, which were then picked up by Sonic's friends. Tails theorized that Chaos had only absorbed the negative properties of the gems and that their positive feelings toward each other could make them work. And so the emeralds were brought together again and Sonic absorbed their positive energy, transforming into Super Sonic. With the emeralds' positive power, Super Sonic defeated Perfect Chaos and calmed its raging heart.
Thanks to Sonic, Chaos was turned back into its former benevolent self. Tikal and a group of Chao approached Chaos and embraced the creature before they all ascended into the heavens.
- Liquid-shifting - Being made of water, Chaos can melt its body down into a puddle of liquid or shape its body into any number of forms.
- Hydrokinesis - Chaos can manipulate any body of water it comes into contact with. This ability is somewhat limited in its basic form but can be enhanced if Chaos consumes a Chaos Emerald.
- Chaos Energy - Chaos can absorb the power of the Chaos Emeralds to become larger and stronger, gaining various other abilities as a result.
Chaos undergoes several transformations throughout Sonic Adventure, changing form with every Chaos Emerald it consumes. Its forms and their abilities are listed below.
Chaos' most basic form without any Chaos Emeralds. This form is vaguely humanoid in shape and attacks by extending its arms to punch enemies. It also performs a spinning attack. Sonic first encounters Chaos in this form in Station Square when the creature is confronted by police outside City Hall.
After consuming its first Chaos Emerald, Chaos grows slightly in height and its right arm becomes longer, developing some form of skeleton on the inside. The Chaos Emerald is embedded in the skeleton hand. It is unknown what abilities Chaos gains from one emerald since it did not have time to demonstrate as Dr. Eggman and the creature fled immediately after Chaos consumed the emerald.
After eating two Chaos Emeralds, Chaos becomes taller and hunches over. Both of its arms have grown a partial skeleton with the Chaos Emeralds embedded in the hands. Knuckles fought against Chaos 2 after following Dr. Eggman into the Station Square Hotel. In this form, Chaos attacked by extending its arms, morphing into a ball and bouncing around the area, and by shifting into a puddle and spreading out over the whole floor. It could also form energy barriers to defend itself, but became vulnerable immediately after using its arm attack.
Chaos 4After devouring four emeralds, Chaos takes on a more aquatic form. Its legs fuse into a fish-like tail and the new emeralds added to its form are attached to boney formations in its lower body. Chaos 4 battles Sonic, Tails and Knuckles in a large pond in the Mystic Ruins, attacking by sending out energy waves across the surface of the water and by swimming toward the large lily pads in the pond and somersaulting to sink them. It could also split its body into four large balls of water which would skim over the pond and attack independently. It made itself vulnerable when it occasionally surfaced from the pond.
Chaos 6Chaos powered up with all but one of the Chaos Emeralds. This form has a huge, hulking body that walks on four squat legs. It also has five eyes, a large mouth and a long, scythe-shaped tail. Sonic, Knuckles and Big the Cat fight Chaos 6 aboard the Egg Carrier, Dr. Eggman's giant airship. Chaos attacks by jumping and creating shockwaves when it lands and can also inhale enemies into its maw. It can also shift into a large amorphous blob and strike by shooting out tentacles in all directions and by attacking with its bladed tail. It is worth noting that Sonic and Knuckles were only able to defeat Chaos 6 thanks to Dr. Eggman's interference, who backed up the monster by dropping small freezing devices from his Egg Mobile. These freeze-mines were meant to immobilise Sonic to make him an easy target, but Sonic grabbed the devices and threw them into Chaos' mouth. The mines then froze Chaos' water body into ice, allowing Sonic and Knuckles to shatter the creature's body and recover the Chaos Emeralds.
Perfect ChaosChaos' ultimate form after consuming all of the Chaos Emeralds. The beast transforms into a massive creature comprised of a dragon-like head covered in spines and surrounded by tentacles. In its perfect state, Chaos is capable of spreading itself over a massive area as it was able to flood the city of Station Square. It is likely that, if left unchecked, the monster would have eventually flooded the entire planet. Perfect Chaos could use its tentacles to cleave buildings in half, could launch its spines like missiles, fire a humongous beam of energy from its mouth and create vortexes around itself for defense. However, this form only wielded the negative energies of the Chaos Emeralds and Sonic was able to harness a positive aura in his Super form, allowing him to tear through Chaos and neutralize the negative power and calm Chaos' raging heart.
In all of its forms, Chaos has an obvious weak spot: a brain is clearly visible in its head. Chaos is immortal so it cannot die, but striking the brain will definitely stun the creature and cause it to collapse into a puddle of liquid. It is also vulnerable to freezing.