From Sonic Retro
How to Use
Chaos Blast appears in two games, but the method to use it is roughly the same. Shadow must trigger his "Chaos Boost" (also known as "Dark Shadow") mode by either destroying enough "dark" enemies to fill a gauge, or purchasing the third "Memory Shard". In Chaos Boost mode, Shadow glows a bright red color and has enhanced abilities.
Once in Chaos Boost mode, the player must either press the Right Trigger or the upper controller button. Shadow will summon energy to himself and release, causing a massive explosion that will damage all targets in a somewhat wide radius of himself. With the exception of humans, smaller targets are completely destroyed, and larger targets lose a significant amount of health. In Shadow the Hedgehog, Chaos Blasts can be used in succession to gain altitude, though normal gameplay only allows for three unless there are quite a few enemies around that where killed by the blast to increase the Dark gauge more. Shadow may also use Chaos Blast as Super Shadow, though it is impossible to tell if the effects of it are increased.
It should be noted that in the 2006 Sonic the Hedgehog, Chaos Blast has a significantly smaller radius than in its debut game. In fact, the radius is decreased so much that the move is virtually useless.
In Sonic Battle, a similar version of Chaos Blast, called "Chaos Burst" was available. It was exactly like Chaos Blast, except it was weaker, had a smaller blast radius, and it was available initially.
In "Sonic and the Black Knight" during the Deep Woods mission Lancelot Returns, Lancelot can use a move greatly similar to Chaos Blast called "Knight of Chaos". When the sword Ddraig Goch is accuired this move becomes useable, changing his Soul Surge from a move like Chaos Control to Knight of Chaos.
Shadow has also been shown to use Chaos Blast in the American Sonic the Hedgehog comic series. There, Shadow activates his Chaos Boost mode by removing his inhibitor rings.