Sirius Black and the Sorting Hat's Retribution

Ever wonder what if it wasn't Harry generation that had to solve the Voldemort issue? What would have happened if a Gryffindor was sorted into Slytherin? What if Voldemort really had wanted the Heir of House Black to be his right-hand man? A story of an early generation ending a war before it starts. An AU of the Marauder age. Good-Slytherin Sirius. No pairing.