Wrong spell (transformation)

"I-I want to fuck you so much!"

"Please, calm down, I messed up the spell, I just need to find the counter spell in the book."

Jane wanted to give her boyfriend better stamina. But things didn't go as expected, she was just a student at the magic academy and the spell was too complex for her. Now, he was in a constant arousal, his dick fully erected in his pants. She picked up another book to find a solution, telling the teacher would give her a huge scolding. But he was already undressing himself and her, that's how she realized that she had now a pet collar.

"Shit shit, what's going on!"

"I want! I want your pussy, Jane!"

"Ok ok, calm down!"

Jane noticed too, that she wasn't really fighting back. She let him take off her clothes and fuck her from behind as she was going through the book. She was clearly affected by the book, but in another way.

Trying to study as he was fucking her was a challenge, her head bobbling with each thrusts of him. Physical changes started to operate on her man's body, his skin gray-green, his teeth sharpen, his hair falling, and his brain losing intelligence.

"Me love Jane's pussy! So soft and warm!"

"Please, please, calm down! I'm... I'm trying to study! I-I can fix... fix it!"

But he wasn't slowing down or calming down. In fact, he was getting better and better with time. It was difficult to concentrate on the notes. She was so close, it was just there, if she could focus a bit...

"Please, m... master! Calm down... I'm close... Aaanh!!! So close to... fuck!!"

"No! Want fuck, fuck pussy!"

He... the one she could now only refer as her master, now, was changing more and more. Becoming more monstrous by the hours... Oh, God, ... hours, they were doing this for hours...

She couldn't do anything anymore, he was barely looking like a civilized being anymore, with his huge teeth, yellow eyes and huge ears. She couldn't refuse him anything, either, despite him being barely able to say more than two words.

"God! Your cock!! Yip! It's too big! AAaanh!!!"

"Fuck! Fuck pussy!!"

There was no chance to do anything, it was lasting for so long now. Her master wasn't letting her go, he was even fucking her ass, now. Somehow his massive member could fit in her tight ass, it was so painful, and yet so pleasurable. Thinking was beyond reach, there was only pain and pleasure.

"AAaannh!! YIP!! Fuck!!! OoOooh!! YIP YIP!!"

"GroooAAaaar!!!"

Jane couldn't stop her owner to run away to the demon realm after both of them were found out in the library. With that, all hope of ever-changing him back was lost. He was just a horny beast, inhuman in look and behavior, while she was almost completely a beast in mind. Language was escaping her grasp, and she couldn't even imagine disobeying him.

"Yip!! Yiip yip! YiiaAAannhip!!"

"Grunt... humpf... Grrahh..."

Caught by the demon lord, she was separate from her owner to be given to a new one, the demon lord itself. It was such a heartbreaking separation. While he was put in a cage away from her, the demon lord was using her as a pet ****.

"Yip! YIP!! Yip yiiip yip!!!"

"Fuck you're so noisy, pet! You look like an elf, but you're way more annoying!"

Jane was used by the demon lord for several years before he got bored, and she was put back with her original owner. They then lived happy in his menagerie, fucking and having a lot of baby monsters for the lord.