Author Note / Disclaimer: I didn't actually write the entire prologue. Some of it is from the end of Prisoner of Azkaban with what I did write being anything you don't recognize. Chapter one is going to start at the beginning of the fourth book and what happened after Harry got knocked out will be explained.
Prologue
Silently they tramped through the grounds, the castle lights growing slowly larger. Snape was still drifting weirdly ahead of Black, his chin bumping on his chest. And then –
A cloud shifted. There were suddenly dim shadows on the ground. Their party was bathed in moonlight.
Snape collided with Lupin, Pettigrew, and Ron, who had stopped abruptly. Black froze. He flung out one arm to make Harry and Hermione stop.
Harry could see Lupin's silhouette. He had gone rigid. Then his limbs began to shake.
"Is he safe?" Hermione gasped. "He never said if he took his potion tonight."
"Run," Black whispered. "Run. Now. He might not be safe."
But Harry could not run. Ron was chained to Pettigrew and Lupin. He tried to get to them but Black caught him by the chest and threw him back.
"Leave it to me – RUN!"
Sirius grabbed Remus by the shoulders. "Remus, my friend, did you take your potion tonight?"
He did not have time to answer because just then his head began to lengthen along with his body. His shoulders were hunched. Hair was sprouting visibly on his face and hands, which were curling into clawed paws.
Sirius held onto Remus waiting until the transformation was complete before letting go and taking a few cautious steps backwards.
Sirius disappeared from Harry's side. He had transformed. The bear like dog moved in front of Harry, standing between him and the werewolf. The werewolf did nothing though.
Harry stood, transfixed by the sight, too intent upon it to notice anything else. It was Hermione's scream that alerted him –
Pettigrew had dived for Lupin's wand. Ron, unsteady on his bandaged leg, fell. There was a band, a burst of light – and Ron lay motionless on the ground. Another bang – Crookshanks flew into the air and back to the earth in a heap.
Sirius, having decided Remus was safe, leaped at Pettigrew from the side. Seizing him by the arm he forced him to the ground.
"Expelliarmus!" Harry yelled, pointing his own wand at Pettigrew. Lupin's wand flew high into the air and out of sight.
Breaking free of Sirius, Pettigrew leapt at Harry, knocking him to the ground. He hit his head on a rock and the last thing Harry saw was Sirius dragging Pettigrew off of him.