The true heir of Slytherin

Salazar Slytherin has finally enough. He can't watch the downfall of the wizard kind lead by Albus Dumbledore and Lord Voldemort any longer, so he decides it is time to intervene. But how is he supposed to change things, when people don't even know he is alive? And what is his connection to the Boy-Who-Lived? Slytherin!Harry, Dark!Harry, Powerful!Harry, Dumbledore bashing, AU