Karl had learned a spell that could transform a human body in any way desired for a short period of time.
He had always suffered from an inferiority complex when it came to sex, because he believed that he had a small penis. His dick wasn't really that small, but he was convinced otherwise.
As such, he knew how he would use the spell. All it required was that he say the magical words, substituting the change he wanted into the blank on the scroll he had copied in class.
Harry warned him repeatedly of the risks he was taking in making such an impulsive spell. Harry knew from past experience that Karl needed to take his time, otherwise his spells always backfired. But Karl was determined to try it, and try as he might, Harry couldn't talk him down.
Harry went and sat at one end of the room while Karl stood in front of the mirror at the other end. Karl stood, naked, with the scroll in one hand, and some lube in the other. Because they were roommates, Harry and Karl had seen each other naked, and were used to it, so Karl's nakedness wasn't a problem. In fact, Harry was just wearing his underwear, so it wasn't much of a difference. Harry pulled out an alchemy book and started reading.
Karl began the spell. The first part of the spell took almost a minute to cast, and Karl came up to the blank part. Karl emphasised "Big Cock" as he gestured at the mirror. He then continued to cast the spell, his eyes on his crotch as he came to the last word - he was already hard, and he thought he could see himself getting ever-so-slowly larger.
Harry, at the other end of the room, felt something strange. He felt compelled to rise to his feet. He looked at Karl, but Karl was engrossed in watching his crotch. Meanwhile, Harry was experiencing forces that were trying to mangle his body. He tried to talk, to yell at Karl, but couldn't muster the breath.
His chest was forced forwards while his arse was pushed backwards. His underwear fell down and his neck craned upwards as his shoulders were forced back. Then he felt it - it started as a slight itch, and progressed to a burning sensation all over his body, except his legs and mouth. The room seemed to get hotter, and he felt sharp stabbing pains hit his rear.
He felt a painful twisting sensation in his feet, and he felt his stomach bloat. Suddenly, his mouth and nose went numb, and something large and yellow entered his vision. His arms pulled away from his body, and then fell back, feeling stiff.
Karl walked over to the table, mumbling "Why isn't anything happening?" He didn't even glance in Harry's direction.
With Karl out of the way, Harry could see himself in the mirror... or rather, he could see SOMETHING in the mirror. He cocked his head, and the giant rooster in the mirror cocked its head, too. Harry's mind scrambled to figure out what was going on.
After a few moments, it dawned on him; he let out an avian squawk, and watched as the rooster in the mirror, that he had become, squawk. He started to go crazy, flapping and squawking, white feathers flying into the air.
The sound caught Karl's attention. He looked up, and nearly fainted with shock. Harry didn't notice as he felt a strong urge. He gave into the urge.
"COCKADOODLEDOO" Harry crowed as only a rooster could, and he just couldn't stop himself. He started scratching at the floor with his avian claws, his head bobbing as he walked around the room.
Karl recovered, and started to talk, but Harry was too focused on his scratching.
Karl realised that it was his carelessness that caused this. He quickly formulated a plan.