This transformation exists primarily or exclusively within the Pre-Super Genesis Wave continuity. Information in this article may not be canonical to the storyline of the games or any other Sonic continuity.
When turning into Chaos Shadow, Shadow's body gains a reddish-yellow hue and a wild energy aura due to the massive amounts of energy that is released. His pupils and irises also vanish.
Shadow was first seen utilizing the Chaos form against Knuckles the Echidna when he was turned into Enerjak. After realizing how powerful his opponent was, Shadow removed his rings and attacked Enerjak at full force. While holding his own for awhile, Shadow was inevitably defeated and left helpless against the demigod until Dr. Eggman intervened.
Shadow used the transformation again against the Doom Laser the evening he, Sonic the Hedgehog and Rouge the Bat infiltrated Eggman's tower to stop it. After their previous attempts failed, Shadow mustered the power of his Chaos form and diverted the beam of the cannon with Chaos Control. The physical strain nearly caused him to fall off the weapon but Sonic caught him just in time.
In this form, Shadow can tap fully into the Chaos Force, granting him an unlimited supply of Chaos energy and turning him into a dynamo of raw, uncontrollable power. This increases all of Shadow's abilities to the point where his Chaos Blast could decimate the entire city of New Mobotropolis, briefly fight back Enerjak, and divert a laser powerful enough to destroy half of the Eggdome.
The Chaos Shadow transformation is only temporary due to overuse of the excess energy which severely drains Shadow of his stamina.