Chaos

Chaos is a Chao that mutated into a water elemental, which was responsible for all but wiping out the Knuckles Tribe of Echidnas. Imprisoned in the Black Emerald by Tikal, a girl from that tribe, Chaos remained trapped for centuries until he was released by the maniacal Dr. Eggman. After accumulating power from various Chaos Emeralds, he attacked the Hidden City of the Ancients only to be stopped by Super Sonic and Knuckles. Following this, Chaos, Tikal and the Seven Server Chao returned to their zone within the Black Emerald.

The Knuckles Clan
Prior to becoming a legendary force of destruction, Chaos was the peaceable guardian of a group of Chao living in the Mystic Ruins. He worked at this task for an unknown period of time, until he eventually met Tikal when the Knuckles Clan settled near his home. The two soon became friends despite Chaos' inability to speak, and for a time all was well. But as the war between the Knuckles Clan and the inhabitants of the Mysterious Cat Country escalated, clan leader Pachamac sought to seize the seven Chaos Emeralds that belonged to Chaos' charges. When he and his warriors attacked, Chaos was corrupted by outrage and destroyed them, going on a rampage that might have spread to the rest of Mobius had Tikal and the Chao not been able to seal him inside the Black Emerald, putting an end to his threat-for a time. (StH: #82)

Release and Evolution
Chaos' legend fell into obscurity, and it was only as Dr. Eggman looked over the file network of his deceased counterpart Dr. Ivo Robotnik that he discovered Chaos' existence and began scheming to claim the creature's power for himself. Using an army of Robians, he excavated the Black Emerald from the midst of the Mysterious Cat Country, cracking it with a hammer blow only to discover that Chaos' imprisonment had weakened him. However, Chaos remained strong enough to lead Eggman to an ancient inscription that revealed to him that Chaos could be restored using the seven Super Emeralds created when the Chao imprisoned him and drained his powers, or by using seven Emeralds equally powerful. When Eggman shattered the Master Emerald into pieces to see his goal fulfilled, a part of Chaos' substance landed in the Mysterious Cat Country and was swallowed by Froggy. Eggman was able to help Chaos on his first step towards recovery by using shards from the Master Emerald as well as Froggy's now-altered DNA to restore some of Chaos' strength. (StH: #79, #80, #81)

Thus fortified, Chaos infiltrated the Hidden City of the Ancients and soon became involved in a battle with the local police. He was then challenged by Sonic the Hedgehog, who defeated him as he was in a weakened state. Later, Chaos attacked Sonic, Miles "Tails" Prower, and Knuckles the Echidna, stealing from them Master Emerald shards and two Super Emeralds in order to evolve into his next form. However, Chaos still proved no match for the three, and was once against defeated and forced to retreat with Dr. Eggman. (StH: #82, SSS: #13)

Chaos was once again able to evolve while on board the Egg Carrier, after Eggman stole two Super Emeralds from Big the Cat. With this new form, Chaos nearly drowned Knuckles within himself, but Sonic managed to freeze Chaos by throwing two of Eggman's Ice-Bots into him. Sonic and Knuckles both shattered the frozen Chaos, which caused a chain reaction and resulted in the Egg Carrier's destruction. Sonic and Knuckles collected the Master Emerald shards and Super Emeralds and fled, while Chaos managed to escape by sticking to Eggman's flying pod. (SSS: #13)

At this point Chaos began working on his own without Eggman. Knocking both Eggman and Knuckles out, Chaos proceeded to absorb the Super Emerald in Tails' bi-plane, which Big the Cat had crashed while escaping from the Egg Carrier. Chaos then broke into Station Square's power plant and absorbed the other six Super Emeralds which had been collected, thus transforming into Perfect Chaos, upon which he launched a devastating assault on Station Square. (StH: #83)

Perfect Chaos vs. Super Sonic
Perfect Chaos flooded the streets of Station Square, forcing the Knothole Freedom Fighters and their ally Nate Morgan to work quickly in order to save many citizens. For all their valiant efforts, they could not deal with the creature themselves, and even Sonic and Knuckles were at a loss. Dr. Eggman returned with a second Egg Carrier, intent on reclaiming Chaos for himself, only for Perfect Chaos to betray him once again and blast his ship out of existence. Tikal then arrived and shared with Sonic a vision of Chaos' past, helping him to understand the threat posed by the creature. Using her powers, Tikal removed the Super Emeralds from Perfect Chaos, delivering them to Sonic so that he could use them to seal Chaos within a restored Master Emerald. Drawing on the untouched positive energy of the Emeralds to combat Chaos' negative power, Sonic transformed into Super Sonic and engaged the massive creature. (StH: #83)

For all of Super Sonic's power, though, he found himself in a deadlock with Perfect Chaos, unable to weaken him enough to bring him down. While the two titans clashed, Knuckles set about finding a way to short-circuit Perfect Chaos so as to defeat him once and for all. Using his immense strength to lift an entire power plant, Knuckles restored its connections and set electricity flowing just as Super Sonic led Perfect Chaos into the midst of the electric wires. Chaos reverted back to his original state, seemingly also made peaceable again as the Super Emeralds transformed back into the Seven Server Chao. Tikal then used her magic to teleport herself, Chaos and the Seven Servers back to their zone within the Black Emerald. (StH: #84)

Powers
As a being composed of pure water, Chaos had the ability to elongate his body to attack enemies from a distance, as well as to change shape in order to dodge attacks or fit in narrow spaces. By absorbing Chaos Emeralds or similar objects of power, he has the ability to evolve into progressively stronger incarnations of himself, each differentiated from his original form of Chaos 0 by a number from one to seven, the seventh also being known as Perfect Chaos.