Disclaimer: The world of Harry Potter belongs to J. K. Rowling. I'm not making any profit off of this story.
Most of the dialogue in the prologue is lifted straight out of PoA. This was intentional and was only done to establish how and where the divergence from canon happens.
"I'm going to stun him," said Lupin. "That's all, I swear."
Harry stepped out of the way. A jet of red light shot from Lupin's wand, and next moment, Pettigrew was crumpled on the floor, unconscious.
They were nearly to the castle when Harry felt the cold.
He and Hermione were walking on either side of Ron, who had been jarred rather severely when Lupin wrenched himself away. Sirius was slightly behind them, levitating Pettigrew and Snape. His hands were trembling, and the deep gashes from his fight with Lupin stood out lividly in the moonlight.
After confirming that Hermione could help Ron on her own, Harry slipped Ron's arm off his shoulders and dropped back to walk with Sirius.
"Are you all right?"
Sirius shook his head. "I was a dog at the time, so there won't be any permanent damage."
"That's not what I—" Suddenly, Harry started shivering violently. A wave of cold swept over them. Harry turned; there was a mass of dementors coming up behind them, hundreds of dementors, faceless, eyeless monsters in black hooded cloaks…
A/N: A little something I've been working on for awhile now, and I finally got the guts to start posting it. And yes, I suck at titles.