Dark Sonic is a Super form of Sonic the Hedgehog and separate entity from him entirely. This version of him was created by Nexter45 for the fanfiction, Sonic: Adventures of the Blue Blur. He was first mentioned in Episode 2.
Dark Sonic is dark blue to black in coloration. He has glowing yellow eyes and his brow line seems more angry. However he always has a devilish grin on his face. His skin is grayer than Sonic's.
Dark Sonic as an entity is completely evil. He has no qualms with killing. He wishes to take over Sonic's body so he can destroy the world. Sonic himself has to be very angry to transform into Dark Sonic and he is able to go into intermediate forms where he is still in control.
After the events of Sonic: Unleashed a small portion of Dark Gaia's energy was left over in Sonic. As time passed all of Sonic's negative emotions and residual Chaos Energy attached themselves to the Dark Gaia energy. This created the seperate entity that is Dark Sonic. After Metal began to encroach on Sonic and his friends, Sonic is visited by Dark Sonic in a nightmare. As the situation degrades, Sonic gets more and more angry and enters an intermediate state, punching a hole in a metal workbench. He doesn't fully transform for the first time until he thinks his friends are dead, at which time he destroys Metal Sonic. Sonic is currently training to be able to control Dark Sonic.
Dark Sonic grants Sonic an increased healing factor. His broken hand is immediately healed after transforming into Dark Sonic. Dark Sonic can't easily sustain damage due to this.
Dark Sonic can do a false instant transmission. Instead of teleporting, his sheer speed allows him to disappear and reappear. His speed also allows him to hit with the power of a 10 megaton nuke.