Dark Sonic

Dark Sonic (Dāku Sonikku) is Sonic's darker form. His first appereance was in the 67th episode of the TV show Sonic X when he sees Cosmo scared and Chris is injured by the Metarex. Sonic uses his anger to achieve this state. While in this state Sonic shows his fury by destroying the two Metarex who hurt Chris.

Unlike alternate Sonic's more destructive super form (as seen in the comics), Dark Sonic seems to be able to recognize his enemies and who he should attack and shouldn't but is noticeably more savage; visciously pummeling two Metarex before violently ripping them apart.

Appearance and colors
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Sonic achieved this form by channeling his rage. As such, "Dark Sonic" is completely consumed by it. He quickly destroys both of the machines the Metarex were using to test Sonic and takes a sadistic pleasure in doing so (indicated by his mad cackling and evil grin). Sonic powers down when he is calmed by Doctor Eggman who says Sonic normally acts much better than this.

The events leading to Sonic's transformation into "Dark Sonic" are paralleled in Sonic and the Secret Rings, resulting in Sonic becoming the extremely similar Darkspine Sonic.

Theories
It is presumed that Sonic also absorbed the energy of the fake Chaos Emeralds he was previously knocked into on the Metarex ship, giving him the power to transform into "Dark Sonic". The fake emeralds are later destroyed and "Dark Sonic" is never seen again, lending credence to the theory. Conversely, this is the only time in the series where Sonic expresses genuine rage, so it is unknown whether or not the Fake Emeralds acted as a catalyst to his transformation or if he accomplished it through pure rage. Another theory is that Sonic absorbed the negative energies of the Chaos Emeralds rather than the usual positive energy.

Powers and Abilities
"Dark Sonic" looks similar to Super Sonic except he has a bluish-black color, a blue aura, and no pupils when he is at maximum power. It's possible that he also had augmented Chaos Powers, as he disappeared several times in his battle with the Metarex, and reappeared right behind them, but this could also be due to him traveling at lightspeed. There yet an other theory that he is turning into some what of the orginal super sonic was in the comics. A derrange monstrous creature that would attack friend and foe alike

Trivia

 * Sonic's black coloration in Super Smash Bros. Brawl bears a strong resemblance to Dark Sonic's partially transformed powering up form.
 * In the Archie Comics, Scourge, Sonic's Anti-Mobius counterpart, once told Sonic that if he had one bad day, he would be like Scourge. This form may have indirectly referenced that statement.
 * It should be noted that the correct term IS "Dark Sonic", contrary to popular fandom belief of "Dark Super Sonic".