Chaos energy is an extremely powerful and dangerous form of energy. It exists in minute quantities within just about everything on Mobius, but the most concentrated source of this power is the Chaos Emeralds. Chaos power has been known to have many radical effects, such as causing machinery to malfunction or causing transformations in living beings. The Drakon Empire discovered Chaos energy and use Mobius' Chaos Emeralds to store it, as the energy is highly unstable on its own.
Creation of the Chaos Energy
Prior to the Great War of Mobius over 8,000 years ago, the Drakon Empire developed an unstable energy source, but the "Chaos" energy was too unsafe to use. In order to find a capsule for the energy, the Drakons supposedly began spying on a number of planets, Mobius one of them. They observed the native echidnas for some time until they knew exactly what they wanted. A military squadron subsequently invaded Mobius and took control of the echidnas' sacred Emerald mines. By digging up and using these precious gems, the Drakons utilised their technology to transfer the Chaos energy into the Emeralds. This not only stablised the energy, but gave them the means to control and use it.
Fearing that the Drakons could transfer their Chaos energy into a thousand Emeralds and conquer the galaxy, Pochacamac stole the seven "Chaos Emeralds" and kept them within his Megopolis City home. The Drakons were furious and chased the echidnas down, meaning to retrieve their Chaos energy. An initial battle arose, beginning the Great War. A number of Prosecutors and Sentinels attempted to steal back the Emeralds but inadvertantly ended up creating the Chaos monster, the product of the unstable Chaos energy merging with a captured Prosecutor fish. Eventually, the Drakons were defeated and the Chaos Emeralds remained on the Floating Island, protected by Knuckles the Echidna.
For some reason, some of the seven Chaos Emeralds began taking on different properties, warped by the unstable energy. The Master Emerald became the largest and most powerful of them all, containing enough Chaos energy to transform both Sonic the Hedgehog and a Metallix into super-powered forms. The Grey Emerald has the status of being the "Control Emerald" and has the power to absorb more Chaos energy. This has become vital on three occasions, allowing Knuckles to remove excess Chaos energy from Doctor Ivo Robotnik's body.
Robotnik was one of the first people to learn of Chaos energy's limitless power and attempted to steal the Chaos Emeralds for himself. He befriended (and tricked) Knuckles and helped him retrieve a second set of Chaos Emeralds, at some point separated from the original seven. Knuckles was able to recombine them but wasn't aware that Robotnik would then absorb the Chaos energy into himself. This newfound power allowed him to grow to an enormous size and even turn Tails into glass. Fortunately, Knuckles revealed that the Grey Emerald, thought to be missing, was in his possession and removed the Chaos energy before teleporting him back down to Mobius. Robotnik's second instance of gaining Chaos powers was much more successful; with the powers of a God bestowed upon him, Robotnik completed changed reality and the timeline, plunging Mobius into misery. Sonic was able to convince Knuckles to use the Grey Emerald on him once more. As Robotnik had become so used to the Chaos energy in his body, he seemingly shrunk to nothing when it was removed, although he had actually been sent to the microscopic world of Shanazar.
Sonic first learned of Chaos energy when he stumbled upon Doctor Kintobor's laboratory. The scientist was using them as part of the Retro-Orbital Chaos Compressor, a machine designed to transfer all evil on the planet into the Chaos Emeralds. Unfortunately, a terrible accident (later revealed to be the machinations of a future Sonic) sent Kintobor tumbling into the ROCC. The collision unleashed a tremendous amount of Chaos energy, transforming Kintobor into Robotnik forever.
Keen to stop this madman from taking over the planet, Sonic defeated Robotnik and collected six of the Chaos Emeralds along the way. He then brought them back to Kintobor's laboratory and showed them to Johnny Lightfoot and Porker Lewis, hoping to find out what to do with them. Due to the Emeralds not being in a cold, stable location, the Emeralds warped back to the Special Zone. The resultant burst of Chaos energy released from the action hit Sonic full in the face, turning him into Super Sonic for the first time. From then on, Sonic turned into this demon when stressed or given too much Chaos energy. This came to a head when he fell into the main shaft of the Emerald Chamber, directly absorbing power from all of the Chaos Emeralds. The resulting Super Sonic was more powerful and maniacal then ever but, more importantly, he was full of Chaos energy and was able to maintain this form for even longer. The Freedom Fighters managed to separate Sonic and Super Sonic, but the Chaos energy remained intact in the dimension. Unable to defeat his nemesis, Sonic and the Omni-Viewer plotted to trap him, eventually keeping him in a time-lock.
Super eventually escaped and created an electromagnetic pulse that shut down all machinery on Mobius. However, this exertion almost completely depleted him of his power and he soon lost all memory of who he was. Dazed and confused, Super Sonic made friends with Madge before moving in with Ebony and Pyjamas. With Pyjamas' help, Super gained his memory but continued to be a pacifist, refusing to tap into his powers in fear of turning evil again. Unfortunately, his Chaos energy well was beginning to run dry, meaning that he would soon die. He was able to track an energy source, realising too late that it belonged to the ancient Chaos creature. Nevertheless, Super leapt inside of Chaos and easily absorbed his Chaos energy, turning him into a demon once more. Ebony was able to recombine him with Sonic and would once again only be unleashed in times of great need.
Believing that her kind friend still remained inside of Sonic, Ebony was cheered up when Sonic stated that he felt a peaceful prescence when "The Power" almost turned him into Super Sonic. When he next did, Ebony siphoned off some of the essence and later forced Sonic to turn into Super Sonic. This dangerous act earned her the wrath of The Order of Magick and she was locked away.
Shadow the Hedgehog
Shadow the Hedgehog is another hedgehog that can use Chaos energy, albeit in a completely different way. One of the sides in the Great War had attempted to create a powerful weapon to turn the tides in their favour, combining the time-travelling Sonic's genes with Chaos energy. However, the experiment was never finished and the incomplete project was abandoned beneath the Emerald Hill Zone. On his travels, Kintobor discovered the machine but was unable to finish what had been started millenia ago. Undeterred, the scientist taught the being history, geography, science and politics, making Shadow the person he would eventually be.
A couple of decades later, Doctor Zachary formed the Syndicate and discovered the Shadow capsule. The elderly echidna had dabblings with Chaos energy several times over the years; his initial encounter with Knuckles saw him use one of the Guardian Robots to absorb the Master Emerald's power, only for Knuckles to destroy it and use the decapitated head as a replacement. Much later, Zachary emerged from his hiding place on the island in time to see "The Power" turn Sonic into Super Sonic. This burst of Chaos energy also had the unforseen effect of releasing Tikal from her prison. Zachary attempted to take control of Super Sonic with Tikal's staff but his relentless speech bored Super into transforming back into Sonic, allowing him to escape and defeat Zachary.
Undeterred, Zachary captured Sonic during a mission to the Death Carrier and used his genes to finish the Shadow project. Shadow, the "ultimate lifeform", was able to use Chaos energy for his own gains, controlling it at will. On his first mission, Shadow was sent to the Special Zone, a region filled with Chaos energy, to steal the Chaos Emeralds. He was interrupted by Sonic and the Chaotix Crew, but forced the Chaos-based Omni-Viewer to transport most of them back to Mobius. Shortly afterwards, the unstable Chaos Emeralds caused the entire dimension to implode and disappear. Shadow's Chaos Control then allowed him to transport Sonic through time, showing him his creation and, once again, displaying Sonic's creation of Robotnik.
Sonic and Shadow confronted one another again on the Death Carrier, but Shadow proved to be at a disadvantage when his supplies began to dwindle. He attempted to return to the Chaos Emeralds, using his reserves of Chaos energy to teleport himself across a gap that Sonic fell down. Shadow's progress was impeded when Super Sonic turned up to pummel him into the ground. Defeated, Shadow used the last of his power to teleport Robotnik to safety and the remains of Shadows disappeared.
Chaos Tap and Chaos Siphon
This destructive ending had been a long-term product of the Syndicate's plan to destroy Mobius. The planet is full of excess Chaos energy and, with the power of the Chaos Emeralds, Zachary had planned to ignite it with the Chaos Siphon, ending the world. Realising this, Sonic travelled to the Drakon planet of Drak and pleaded with them to help prevent the catastrophe. Master Scholar was well aware that losing Mobius also meant losing the chance to retrieve the Chaos Emeralds, so Sonic was provided with a Chaos Tap, a device that would absorb all excess Chaos energy on the planet. Sonic returned to Mobius and planted the Tap in the ground. It instantly went to work and had the unexpected effect of causing the Siphon to backfire, destroying both it and the Death Carrier. Vichama then used the Chaos Emeralds to free him from his echidna shell.
The Chaos Tap remained a threat long after the Syndicate disbanded. Robotnik returned to full power and built his base around it, threatening Sonic that he would detonate the Chaos Tap should his plans fail. He also bragged of his plans to Knuckles, prompting him to launch an operation to disarm it. Sonic and Knuckles teamed up to remove it, with Knuckles using the Grey Emerald for the third time. This time, the Chaos energy from the entire planet made the Emerald grow enormous in size, but the Chaos Tap was no longer a danger to Mobius.