"You shall achieve destruction where you stand,
In intimate conflict, at your brother's hand."
-Edna St. Vincent Millay
Prologue: Your Brother's Hand
"Lily and James!" Peter shouts. "Sirius, how could you?"
It is the note of actual reproach in his voice that stops Sirius dead in his tracks. The curses he's been waiting fifty miles to use die unspoken on his tongue from shock alone, and in that instant of immobility, Peter moves with a speed and a purpose that Sirius finds as unfamiliar as the sneering, triumphant expression on Peter's face. Before Sirius can recover himself, there is a roar of noise and sound and he staggers as the ground rocks beneath his feet. Dust and flame obscure his vision. Someone is screaming, voicing the disbelieving howl of loss and pain that has been stuck in Sirius' throat since he saw the ruin of Godric's Hollow. Through the smoke, Sirius catches a glimpse of Peter, form twisting and shrinking, but the Animagus transformation is nearly instantaneous, and before he can raise his wand again, Peter is gone, and Sirius is standing alone in a broken street full of dying Muggles.
For long moments, he cannot move. The sound of sirens rises in the distance, a screaming electronic counterpoint to the sobs and shrieks of the injured and bereaved. Sirius can barely hear it over the voices ringing in his memory: James, saying goodbye just before going under Fidelius; Peter, swearing so earnestly to keep the faith, and his own disbelieving shout of denial when the words Godric's Hollow dropped cold and leaden into his skull as James died and the Fidelius charm was broken. He wants to scream; he wants to cry, but something bright and jagged catches in his chest and he realizes that he's laughing instead, laughing like his heart will break, and he doesn't think he will be able to stop.
It is Peter's finger that finally breaks through to him. His eyes catch on it and cling, even though at first he doesn't realize what he's looking at. But Peter broke that finger in third year, playing chicken with the Whomping Willow, and he never went to Madame Pompfrey so the tip was always a little crooked, and Sirius hadn't known it until now, but he'd recognize that finger anywhere. He stops laughing as the knowledge of exactly what Peter has planned for him penetrates the fog of grief and rage clouding his mind. He'd said it himself. No one will ever believe that Peter Pettigrew was the Potter's Secret Keeper.
He snatches up Pettigrew's finger with a grim smile, and as he Apparates, hears the crack of the first arriving Auror.
It takes nearly half an hour to empty his vault at Gringott's, and Sirius spends the entire time listening for the shout that will mean he is caught. It is impossible to Apparate to or from Gringott's, and if they do find him here, he's finished. He's not certain they will think to look for him here, but he doesn't want to take any chances. It's lunacy to stop off at Gringott's, but Sirius is sure that it would be madder not to. He won't get very far without money, even as Padfoot. Remus will certainly tell the Ministry that Sirius is an Animagus. Sirius pushes thoughts of Remus aside and concentrates on making his fortune pocket-sized.
No one glances in his direction as he crosses the lobby of Gringott's, but some instinct makes him pull out his wand as he pushes open the door. He gets less than five feet from the steps before that same instinct tells him to duck. He turns as he does so, and the curse misses his left ear by less than two inches.
"Black!" his assailant shouts, in tones of such fury that Sirius barely recognizes the voice as belonging to Kingsley Shacklebolt. Shacklebolt sends another curse at him, and Sirius deflects it without thinking. He tries to Apparate even as he hurls Petrificus Totalus at the Auror, and finds that he can't.
"You're surrounded, Black!" He is. Sirius can see three other Aurors, and knows there will be more coming.
"Omnis Obliviatas! Omnis Petrificas! Omnis Stupefactas!" He knows as he shouts that there will be no coming back. The Omnis alteration of spells to affect groups of people has been forbidden for centuries, and using it on a group of Aurors is enough to earn him time in Azkaban. That he has used it at all will be seen as proof of his allegiance to Voldemort. So he runs. While the Aurors are still reeling from the triplicate of curses, he takes to his heels; turns the first corner he comes to, then the second, and has changed from man to dog in the space of time it takes to glance behind him. When the Aurors pass him, he is curled up on the porch of Madame Malkin's, and because he and Padfoot have the same eyes he has to lie still and listen to them pass, while both canine and human instincts scream at him to keep running.
As soon as they are gone, he slips off the porch and into the nearest alley. He shifts back to his own form, and Apparates an instant later.