Disclaimer: This is only a crossover story based on specific events in Harry Potter and the Prisoner of Azkaban. Any inclusion of characters irrelevant to the story is entirely my choice, so don't blow up in the reviews about it!
As the Hogwarts Express started to leave the station, the wizards and witches gathered their things and rushed on board. Harry Potter and his friends, Hermione Granger and Ronald Weasley (commonly shortened to Ron), were the last ones to come aboard. They came across possibly the only somewhat empty compartemotion the whole express, which happened to be located directly next to Pit and the other smashers, as well as the first transfer student in Hogwarts history.
Ron, on the other hand, was paying no significant attention to the occupants of the compartment next door and was instead laying his gaze on the teacher sleeping in the area with them. "Wonder who this is...?" he asked nobody in particular, but his question was quickly answer by Hermione.
"Professor R. J. Lupin," she read off of his briefcase before sitting down with Crookshanks. After receiving a puzzled look from not only Ron, but also Harry, she explained in quick succession, "It's written on the side of his case."
As the Hogwarts Express chugged along, Harry thought it best to explain the matters at hand. By the time he was done, Ron seemed as white as a ghost. "Lemme get this straight: Sirius Black has escaped from Azkaban to come solely after you?" he asked worriedly, gripping Scabbers close to him.
When Harry nodded, Hermione piped in. "But they'll catch Black one day. I mean, everyone's looking for him," she said, trying to keep her mind on the positive side of things. Ron was still weak and shivery, though.
"Sure...except, nobody's ever broken out of Azkaban before, and..." Ron paused for a moment before continuing, "he's a murderous, raving lunatic." Harry rolled his eyes at his sometimes moronic friend.
As he was about to make a sarcastic remark, however, the train suddenly started to jut forward and then back again, then stop entirely. "What's going on?" Hermione asked, now clearly worried for herself. "We can't possibly be there yet." At this, Harry poked his head out the door of their compartment and for the first time saw the smashers in the room beside his own. The train lurched again, though, and sent everyone back into their compartments and slammed their doors shut.
Ron looked outside the window, his hand pressed against it, paying no mind to accidentally stepping on Hermione's foot ("Ouch, Ron, that was my foot!" she had called out in pain). "There's something moving out there, and I think it's coming aboard!" Almost on cue, the window and train car suddenly became surrounded by a thin sheet of frost, quickly spreading to the others and even to the bottle of butterbeer right beside Lupin. When the train lurched a third time, Ron was sent flying back into his seat, and he shouted worriedly, "Bloody hell! W-What's happening?!"
At this comment, a few faint whisps of black were seen. Ron glanced next door for a split second, then turned his attention back to the figure approaching their compartment, which had gotten significantly closer and was now using telepathic power to open the door without touching it. The figure revealed itself as a large, black, hooded creature with a large hand gripping firmly the inside of its cloak. It gave a long stare at the group before drawing in a long, slow, rattling breath in Harry's direction. Hermione and Ron noticed this and tried to pull him back, but to no avail. The smashers rushed over and helped as well, trying to knock the creature back. The creature simply shrugged the attacks off and continued its process. Quickly and suddenly, Lupin's eyes shot open, and his drew his wand, muttered something under his breath, and shot a large, glowing sphere at the creature, which drove it back. The last thing Harry heard was a female's voice screaming his name, in rising panic, before all of his senses suddenly went dark.
"Harry!" Harry could feel someone calling out to him, out of the blue, from the blackness. He slowly started to open his eyes. "Harry, are you alright?"
As Harry sat up groggily, his head spinning, he saw the source of the voice calling out to him: Hermione. He also noticed the kids from the other compartment in his own, and he felt at his eyes to try to adjust his glasses, which were missing. Hermione handed them to him. "Thank you," Harry muttered under his breath, putting his round glasses back on.
"Here, eat this. It'll help." Harry looked up to see Lupin sitting over him, handing him a piece off of a slab of chocolate. Harry took the chocolate slab but lacked to taste it. "It's all right, it's chocolate."
"W-What was that thing...That came?" Harry asked nobody in particular as he sat up fully and saw the smashers and transfer student standing over him. Lupin adjusted his cloak and set the bar of chocolate on the seat.
"It was a dementor," explained, looking toward the door with his eyes darting to the left. "One of the guards of Azkaban. It's gone now. It was searching the train for Sirius Black." He stood up and headed toward the door. "If you'll excuse me, I need to have a little word with the driver." As he stood up, he stopped at the turned to Harry, smiling. "Eat. You'll feel better."
Hermione picked up Crookshanks as Lupin slammed the door shut. Harry was curious as to what had happened and, more importantly, why another kid was still unconscious on the floor. Harry stood over the unconscious kid, the same way Hermione had stood over him, and lightly nudged him with his shoe. To his surprise, the kid started to come to on the spot. He looked up at Harry with his mahogany red eyes, and Harry himself looked out the window.
A/N: Ooh, cliffhanger. If anyone can guess who the unconscious guy is, l'll update sooner.