Dark Sonic

When Sonic first crashes into the Metarex's base in episode 67, after seeing how many fake emeralds there are, he feels uneasy. These comments are prompted by the negative energy Sonic can feel from the fake emeralds. It is this dark Chaos Energy that incredibly intensifies Sonic's rage at seeing an unconscious Chris and a terrified Cosmo trapped, that causes him to turn into Dark Sonic in his mid-transfomation form.

Black Narcissus then released two robots, Gold and Silver, he designed to test Sonic's strength and speed. Dark Sonic seems to take a murderous pleasure in doing so, indicated by his mad cackling and evil grin beforehand and quickly tore them apart which then it shows him in is full transformation form. Just then, Doctor Eggman, who declares he has never seen Sonic so worked up before. Eggman says he does not want to see Sonic act out of anger and rage and how it is not good for him as Sonic powers down to his normal self.[1]