I'm not perfectly happy with this chapter, but it will have to do.

The idea is supposed to be that Sirius' fear is that the story was right, and he did actually betray the Potters.

My original thought was that the boggart would show himself with the dark mark, remembering and confessing his guilt. But I didn't want another scene with the boggart as a character. I hope this worked instead. Edit: I just finished writing it when I realised I could have done the boggart as Scabbers being just a rat…

Sirius Black, the notorious 'mass murderer', pads noiselessly through the halls of the school that had once been his home. His dark fur blends with the shadows, allowing him to focus on his goal. Find Peter, protect Harry. The familiar passageways bring back forgotten memories, threatening to overwhelm him, but he pushes them aside. (Find Peter, protect Harry.)

Suddenly, an achingly familiar scent breaks his concentration, blanking his mind. He turns to the closed door beside him and breathes deeply, the scent filling his senses. Moony. He is here. Oh, not here in this room – the intensity of the scent comes from frequent recent presence not current occupation – but in the castle. Currently he is most likely in the Great Hall; he remembers hearing the echoes of a feast as he crept into the school.

He freezes as the pieces suddenly slip into place. Late autumn. Hogwarts Feast. Halloween. That night. A wave of grief-pain-rage crashes through him. James, Lily, dead. Peter, the traitor. (Find Peter, protect Harry.) Find Peter. Find him, rip him, kill him! He snarls, the now-familiar red haze settling over his vision.

But then he breathes. And the scent invades once more, washing away the anger and leaving only the pain. He whimpers, suddenly needing comfort more than revenge. His friend is out of his reach, but there is a room full of his scent right there. Quickly checking up and down the hallway, he transforms and slips inside the room, closing the door behind him.

He looks around, relaxing involuntarily. The office is just so Remus. Shabby and a bit cluttered, but organised, with books and papers and tanks of curious critters piled on every surface. And the whole room is permeated with his scent, detectable even in his human form, and barely changed from his remembered school-days.

A wave of dizziness sweeps over him, memories swirling in broken fragments. Unbalanced on two legs, he stumbles, tripping over a case to collapse on the floor. A thud echoes around the room as the case lands on the floor beside him. Then a faint whispering sound. Tendrils of silvery fog creep around the opened latches, rising to coalesce into three forms. Three very familiar forms, standing over him as they have done in so many dreams. Then, they speak.

"You betrayed us. You were my friend, and I trusted you. I should have known that you couldn't be more than a Black, a Death Eater. I trusted you, and you betrayed us."

James. No, please. I tried to protect you, I really did. It was a mistake. I failed. But I tried.

"Where's Harry? You took him, then abandoned him. We made you godfather, you promised to protect him. But you took us away, and abandoned him. Where's Harry?

Lily, Lily, I'm sorry. I meant my promise, but I couldn't. I thought he was safe, then I found a way to come as soon as I knew he wasn't. (Find Peter, protect Harry.)

"You killed me. You framed me for your treachery. You murdered me to hide your guilt. You killed me."

Peter, I…

No. Nonono. You're alive, I saw you. I didn't kill you, not yet. You betrayed them, you killed them. I won't let you hurt Harry too. (Find Peter, protect Harry.) You're alive. I tried to protect them, but you framed me, and you were our friend, and you killed them, and you will pay. You are alive; you're the traitor. It's all your fault. You did this. (Find Peter, protect… Find Peter.)

He shakes himself, ignoring the ghosts and pulling himself back to the present. (Find Peter.) Carefully unclenching his fists, he focuses on returning them to paws, the rest of his body following suit. Hands are no use now; he needs Padfoot's nose. (Find Peter.) The door to the corridor is nudged open easily and he breathes deeply, searching for faint traces of scent as he follows the once-familiar path upward. (Find Peter).

Tonight he will finish what was started twelve years ago. Peter is alive. Peter is the traitor. It was all him. Peter has to suffer as those he betrayed suffered. (Find Peter.) He allows the red haze to settle over him once more.

(Find Peter, kill Peter.)