Like it's name, it is an explosion of highly powerful negative Chaos Energy that damages anyone caught in it's range which is different between games. In most games, Shadow can use it after charging the Chaos energy inside his body and firing it outside, which really weaken if not destroy the enemy. Just like the other Chaos Powers, it can be charged with every Chaos Emerald in hand, and with the seven, Shadow can use it to its full potential.
Shadow the Hedgehog
This move was introduced in Shadow the Hedgehog. In this game, Shadow can only perform Chaos Blast when in his Dark Shadow form, which he can only enter by filling up his Dark gauge by committing any acts of evil. When used, Shadow unleashes a massive shockwave that destroys everything within its range and makes any enemies that it comes in contact with lose a significant amount of health. In the boss fight against the Egg Dealer, if Shadow lined up three Shadow icons on its slot machine, he would activate Shadow Fever and instantly fills the Dark Gauge to the maximum, allowing usage of the Chaos Blast. The Chaos Blasts can also be used in succession to gain altitude, though normal gameplay only allows for three unless there are larger amounts of enemies nearby that are killed by the Chaos Blast to increase the Dark gauge more. Chaos Blast can also bypass shields like Diablon's and damage him making it really powerful in combating with bosses.
Sonic the Hedgehog (2006)
In Sonic the Hedgehog, Shadow can use the Chaos Blast when he activates the Chaos Boost Level 3. In this game, the Chaos Blast is a small blast of red energy in the immediate one-yard area around the user, which can only be used while in the air. It does not work unless you are very close to the enemy, and will destroy any regular enemy with one hit if he was guarding or not.
Sonic Chronicles: The Dark Brotherhood
Shadow delivers an explosion of chaos energy that may weaken affected targets.
In Sonic Chronicles: The Dark Brotherhood, Chaos Blast is one of Shadow's POW moves. When performing Chaos Blast in this game, Shadow walks up in front of the opposing team, jumps up and fires the Chaos Blast in the form of a purple beam from the palm of his hands at his opponents, before retreating.
In gameplay, Chaos Blast deals attack damage to a chosen target. When dealing damage, Chaos Blast inflicts Blast damage to nearby foes, and also has a chance of inflicting weaken on the target. The higher the level of Chaos Blast is, the more attack damage it will deal and the greater the possibility that it will inflict weaken on the target.
Quick time event
When performing Chaos Blast in gameplay, the player has to complete a series of real-time events by performing certain actions on the Touch Screen, in order to execute Chaos Blast optimally:
The player has to touch the inside of six circles in succession when the outer circles overlap the inner circles.
The player has to tap an inner circle that appears eight times before the outer circle overlap the inner circle
Missing or not touching these circles correctly will either result in Shadow missing the strike or the damage from the attack being reduced.
During the mission "Lancelot Returns" in Deep Woods, where the player has to defeat Lancelot, Lancelot can perform the Chaos Blast, which not only damages the player upon contact, but also briefly acts as a shield after it goes outwards a few yards blocking the player from attacking Sir Lancelot for a short of time.
Sonic & All-Stars Racing Transformed
In Sonic & All-Stars Racing Transformed, Chaos Blast is utilized as Shadow's All-Star Move, replacing Super Shadow from the prequel Sonic & Sega All-Stars Racing. It appears as a short, charged shockwave of red energy that evaporates in a few seconds, but will inflict damage to any racer or obstacle caught in the vicinity of the blast or its remaining flares. Chaos Blast can be used consecutively, but will require charging before it can be used additionally. Once the All-Star Move ceases, Shadow will be unable to use Chaos Blast again until he obtains another All-Star Move from an item box.
Sonic Boom: Rise of Lyric
Chaos Blast also appeared in Sonic Boom: Rise of Lyric. When using the move, Shadow will says "Chaos Blast!" and then charges his attack afloat on the ground and then unleashes an explosion of energy that breaks the ground and deals significant damage to the player.
The Chaos Blast from Sonic and the Black Knight is also known as the "Aura Bubble" as it is more of a bubble than an explosion.
Interestingly, in Sonic and the Black Knight, when Sonic hits Sir Lancelot using Soul Surge in the mission Lancelot's Return, Lancelot glows red and, after a few hits, will perform a Chaos Blast, meaning that he might be charging up to perform this attack while he was being hit.
In Sonic and the Black Knight, Amy has a similar move called Rose Typhoon which is a giant pink energy sphere. It is unknown what this sphere is made of.