Por que Sirius veio para a sala da Grifinória?

Ele estava procurando por Peter Pettigrew.

O propósito de Sirius ao sair de Azkaban e ir a Hogwarts foi matar o traidor que causou a morte de seus amigos e sua própria prisão. É por isso que ele continuou murmurando "Ele está em Hogwarts", mesmo enquanto ainda estava em Azkaban (levando as pessoas a pensar que ele estava caçando Harry para matá-lo ).

Ron was sitting up in bed, the hangings torn from one side, a look of utmost terror on his face.

“Black! Sirius Black! With a knife!”

“What?”

“Here! Just now! Slashed the curtains! Woke me up!”

_{-- HP and the Prisoner of Azkaban, Chapter 13: Gryffindor vs Ravenclaw}

E mais tarde, o próprio Sirius relata como ele invadiu a Torre da Grifinória, com a ajuda de Crookshanks, a fim de encontrar Pettigrew , mas falhou:

“[Crookshanks] tried to bring Peter to me, but couldn't... so he stole the passwords into Gryffindor Tower for me... As I understand it, he took them from a boy's bedside table...”

Harry's brain seemed to be sagging under the weight of what he was hearing. It was absurd... and yet...

“But Peter got wind of what was going on and ran for it.” croaked Black. “This cat—Crookshanks, did you call him?—told me Peter had left blood on the sheets... I supposed he bit himself... Well, faking his own death had worked once.”

_{-- HP and the Prisoner of Azkaban, Chapter 19: The Servant of Lord Voldemort}

Sirius Black chegou à Torre da Grifinória para matar Peter Pettigrew.

Sirius descobriu que o rato de estimação de Rony Weasley, Scabbers, era na verdade Peter Pettigrew escondido como sua forma de Animago.

"You know, Sirius, that's a fair question," said Lupin, turning to Black and frowning slightly. "How did you find out where he was?"
Black put one of his clawlike hands inside his robes and took out a crumpled piece of paper, which he smoothed flat and held out to show the others. It was the photograph of Ron and his family that had appeared in the Daily Prophet the previous summer, and there, on Ron's shoulder, was Scabbers.
"How did you get this?" Lupin asked Black, thunderstruck.
"Fudge," said Black. "When he came to inspect Azkaban last year, he gave me his paper. And there was Peter, on the front page on this boy's shoulder... I knew him at once... how many times had I seen him transform? And the caption said the boy would be going back to Hogwarts..."
_{Harry Potter and the Prisoner of Azkaban Chapter 19: The Servant of Lord Voldemort}

Peter Pettigrew traiu os Potters para Voldemort e emoldurou Sirius por isso. Sirius encurralou Pettigrew em uma rua trouxa após o incidente e Pettigrew explodiu a rua, transformou-se em um rato e escapou, matando muitos trouxas. Sirius foi considerado culpado por trair os Potter, matando Pettigrew e todos os Trouxas e enviado para Azkaban. Sirius escapou quando percebeu que Pettigrew estava vivo e veio a Hogwarts para vingar a traição dos Potter.