Dark is one of Sonic's dark Forms, he is basically Sonic's sealed negative emotions, that has manaphessed itself as a being, Dark, is Sonic's rage and revenge emotions.

You were at Tails' house, since he called to help him with a problem that he had with Sonic, but the call had ended by the sound of Tails choking, you quickly rushed over to see if Tails was okay, opened the door, to find That Sonic was holding his friend by the throat.

"How dare you? I thought I could trust you, but instead you back stabbed me, letting him do those things to me" Sonic said before he through Tails at the wall.

You.....................................