This is a collaboration with  PortalJaguar2. Here's the story!

Don poked his head through the door and let out an echoey “Hello?”
“Dude, no one’s in there. Just go in!” Luke pushed the door open a little wider, causing Don to fall in.
“Yeah, I know!” He said confidently. Still, there was a little quiver in his voice.
The two had decided that it was time to check this place out. They had passed it several times in their life, and it was even on the corner of the block where Don lived, but no one seemed to live there. It was such a large house, it got to the point where curiosity took over, and Luke was determined to see what was inside.
“Are you sure about this? I mean maybe someone still lives here.” Don dusted himself off from the landing.
“I’m pretty sure no one lives here.” Luke retorted. They entered the very creaky wooden house. The place was humongous. It had a chandelier and two sets of stairs. It felt like it hadn’t been dusted in ages, which caused Luke to sneeze. “Yeah, pretty sure,” He confirmed. It only took a few steps for him to feel like he had cobwebs in his hair. “Upstairs or downstairs?”
“Let’s look downstairs first.” Don said, looking around. “I’m worried those stairs will cave in any minute.”
“I think you’re right.” Luke dusted the cobwebs from his hair. They entered the kitchen, which had all sorts of rotten food and trash scattered about. The was even a cluster of rats. “Ugh!” Don shrieked as one of the rats came up to him, “GET IT OFF!” Don jumped and screamed. Eventually the rat scurried away and Don calmed down.
Luke looked at him worriedly. “It’s just a rat! You’ll be alright.”
Don shook his head violently. “No, that’s just gross! Eww!”
Luke rolled his eyes as he watched another one scurry across their path. “They’re probably just attracted to the place. I mean, there’s food everywhere.” Don just shivered.
They continued on their “Journey” as they walked away from the kitchen. They entered the living room where there was twice as many cobwebs as before. “This place is giving me the heebie jeebies.” Don squeaked. Luke turned on his flashlight as the room got darker.
“It is a little creepy. I bet there’s ghosts!” He teased Don. Luke didn’t believe in ghosts himself, but it was fun to mock and see if he could get a reaction.
“Don’t even joke about that!” Don said, his nerves obviously on edge, “I’m freaking out!”
“Sorry, sorry!” Luke laughed. The place was creepy, but he had to have some fun with it.

They began to walk into what looked like a basement. Don shoved Luke “You go. It’s too creepy in there. I’ll be behind you.”
“No, you go.” Luke shoved him back. They argued for a minute until they heard a loud THUMP! Don’s heart began racing “What was that!?”
Luke looked around, then back at Don. “It’s probably just a rat or something knocking somethin’ over.”
“I don’t know, dude. I think it might be a ghost.” Don shivered.
“I doubt it.” He turned around and started going into the basement, deciding to take the lead.
But in the back of Luke’s mind he couldn’t help but think that maybe it was a ghost. Or maybe it was something more sinister.
The stairs squeaked as they went down into the basement. Cobwebs were everywhere, along with rats that made Don shiver. “I don’t like it down here.” He said.
Luke, trying to be as brave as he could, said “It’s fine.”
When they got down they couldn’t believe what they were seeing. There were potions on shelves, eyeballs in jars that followed their movements, and a giant pot in the center of the room. “Let's get out of here.” Don said, his nerves on edge. Don turned around and bumped into something. Or someone.
“What are you doing down here!?” Asked an old lady. Don shrieked with fear while Luke was frozen in place. She cackled and told them “Have a seat my dear!”
Luke looked at her in shock before shaking his head. “No, no… I’m good. We didn’t realize someone lived here. We’ll be on ou-”
“No, no. I insist.” She smiled coyly. She wiggled her finger and a chair shot forward, knocking Don down into it. He tried to stand, but the chair floated in the air, and he found himself clutching the seat tightly, his knuckles turning white. He looked at the chair, then the witch, then finally at Luke in a look of panic.
“Wh-what th-the hell?” Luke shook slowly, backing away. Another chair shot forward picking Luke off the ground. She cackled again looking at them. Don looked at Luke with shock.
“Is she a witch?” Don asked.
“Oh, sweetie.” She looked up at Don, who looked back at her in terror, “I thought that was obvious!”
The two chairs danced in the air, Luke holding on just as tightly as Don. He glanced around, in a desperate hope to find a way out of this, but there was nothing near. He could feel his heart beating.
“The question is, now, what to do with you two? Options, options!”
Don shook his head. He didn’t like the sound of that. His chair lurched forward, and he winced.
“Just leave us be!” Luke said flatly. He sure wasn’t feeling brave, but he pushed himself to call her out. He just hoped she would listen to him.
“Oh, courageous one, aren’t we?” She waved him over, and his chair jiggled in her direction. “I can fix that. You’re strong, tough. Confident. Even a little blunt. Perhaps something a little more timid will do for you. And you.” She looked at Don and put her finger on her chin. She looked at him, surveying every part of him.
“Thin, lots of running, and you just seem terrified!” She said. “I think I know what to do with you.” she walked away.
“What? What are you going to do to us?” Don and Luke spoke at the same time.
“You’ll see, you’ll see!” She smiled cheerily. The witch faced away and opened up a large, dusty book. She called Don’s chair over with another wave of her hand and looked him in the eyes. In some unknown, foreign language, she spoke, and never broke eye contact. Don looked like he was in a trance, his eyes wide and his other senses dulled.
“Don’t listen to her! Snap out of it!” Luke tried calling out. It was in vain. He was hypnotized by her words.
“I’ve been in need of a new familiar,” she ended her spell, and lowered his chair, “and I think you’ll do quite nicely.”
Wait, what is she talking about? Luke thought. She lifted up Luke’s chair and began chanting some strange words. Words that made Luke’s hairs stand on end.
His chair slowly crawled towards her, and his eyes fell into a trance much like Don’s had earlier. He could feel his heart racing further, as if he knew what was happening, but he was powerless to stop it. She finished her spell and lowered his chair next to Don’s.
She just smiled at them sinisterly.
It started with their clothes. It was as if someone had put a match against their shirts and just let them burn. Smoke filled the air as Luke and Don tried desperately to dowse the invisible fire, but it was to no avail. Their shirts burned off, then their pants, shortly following their shoes, and then finally, their socks. They were left there in their underwear, exposed, and a bit embarrassed.
“What the hell?” Don exclaimed, looking up at her.
“I need to make sure to see this through. With pesky clothes, it’s possible I missed a spot! Casting spells isn’t easy, you know!”
Luke felt a shiver run up his spine, then swiftly back down. This time, however, the shiver erupted out, and he felt something peek out from just above his boxer briefs, out of the small of his back. A thin, naked tail began to wriggle out, and he felt it all.
Don let out a surprised yowl as the same happened to him, his own tail bursting from the small of his back. His, however, was much furrier, and flicked in rhythm with his heartbeat.
“TAILS!?” Don screamed out, “WE HAVE TAILS!?”
Luke grabbed his and held it. It was long and thick, and felt a bit leathery. He looked at the witch. This all felt so wrong.
“Now, you,” she pointed to Don, “you’ll be well taken care of. You’re just scared. But you!” She pointed to Luke, “you’ve treated me so poorly! You’ve been quite a pest.”
Don shook his head. “No, please, leave him alone!”
“Don’t worry. I’ll let him live!”
Luke’s eyes widened. He supposed that was good news.
“But he’s still been quite a rat.”
“Rat?” That’s when it clicked.
She smiled. She could see the realization cross his face. It was then the next phase of his change occured. He could feel his feet crack and bend. He fell out of the chair and onto the floor in surprise and he began to examine them. While normally tan, his feet had become quite pink, and his toes had lengthened out, sharp claws protruding from each digit. He could hear them crack as they reshaped. Gray fur began to protrude from his ankle, contrasting his new pink feet wonderfully.
She turned her attention away from Luke and looked back at Don. “I hate to see you look so worried.” It was clear she found Don to be her favorite. “Perhaps…”
Don wiggled his nose. His face had begun to sprout black fuzz, his nose becoming pink and flat. After watching what was happening to Luke, he knew what was about to happen to him.
“Please, no! I don’t want to be a rat!”
“A rat? Oh, no. Sweetie. No.” She shook her head, amused. “You won’t be a rat.”
Don let a small sigh of relief, but was still shaking with fear. Don’s face began to extend outward, his teeth beginning to shape themselves into a sharp point. Don poked at his teeth and felt the sharp canines come in. “What am Irrrraaaarrr?” Don tried asking.
“You are going to be my familiar.”
“Whyrrrrr.” Don struggled to keep his voice human. Whiskers began to sprout above his lip. He touched them feeling the shock of the nerve as he pulled it. She laughed an evil laugh and turned to Luke.
“I enjoy watching you writhe around. Your panic amuses me.” Luke looked up from his new paws for a moment to see her looking down at him bitterly. Gray fur began to sprout from his legs, climbing up his shins and calfs from his ankles, causing him to look away. She continued to laugh as he scratched madly at the growing fur. His feet, which were now fully clawed and pink, began to extend, growing longer, more bones cracking. The sound was shocking, and it caused him to grow dizzy for a moment. The changes finished, and he tried to stand. The witch pointed her finger down and he quickly dropped back down onto the ground, landing on his tail. He let out a yelp of pain. He hadn’t expected that to hurt like it did.
Don’s hair began to fall out making room for little black fur. He felt his ears began to crawl up to his head. They flicked as he turned to look at Luke, who was laying on the ground in a heap. Don stood and headed towards him, but a force held him back and he found himself sitting back in the chair. “Oh, you can leave him be!” The witch said, her eyes glimmering with malice, “I have plans for him! As for you,” she rolled her head to the side. The black fur began to grow down his neck. While he couldn’t see it, he could feel it. It rolled along, slowly covering his shoulders and his chest. He felt one of his feet change. His toe shriveling up to form a paw. He tried to exclaim, but all that came out was a broken meow. She smiled. “See, I knew you’d just be precious.”
“You monster!” Luke scoffed.
She laughed and came over to him. “I’m not mean. You just deserve what you get for going into my house.”
Luke felt like things were getting bigger. The lady seemed a bit taller than she had been before. That's when it came to him. He was getting smaller.
“Don’t want you to be a giant rat.” She laughed. Her laugh pierced Luke’s ears. The grey fur was now spreading onto his knees and along his thighs. So this is what we get for sneaking into other people's property, Luke thought. He turned to Don and saw he was in mid transformation.
Don let out a couple of meows, but cut off after a few. He knew he couldn’t say anything. She had taken away his ability to speak. The fur was traveling down his arms, his hands writhing as they too changed shape, much like his head. His fingers curled as his nails became sharp claws, which he could retract at will. His palms and fingertips began to swell, pink paw pads forming. His tail flicked in fear as he watched it all happen, knowing he couldn’t stop it.
Luke was at least 4 feet tall now. He used to be 5’4 but had lost that extra height. He stood up and wobbled to stay standing on 2 legs. The fur came up to his waist and made him itch madly.
Luke felt his ears come up to the top of his head. Two big, round, weird looking ears. Luke squeaked, which was a bit odd. He reached up to feel them. They were soft, and they felt massive. He looked at the witch, who just stood there, smiling. He not only looked different, but he was beginning to feel different. He felt more frightened. Weaker. More vulnerable. The witch; she terrified him. He let out another squeak as he tried to back up, but the witch shook her head, and he found himself frozen in place.
Don watched as Luke stood there, the witch walking towards him, but he had his own things to worry about. His arms and hands were now completely feline, the black fur creeping down his stomach. He tried to grab it, in a meaningless desire to stop its growth, but it continued its march down his torso.
Don jumped down from the chair silently and looked for something to stop the witch. Don found a green potion. This looks like it will do, Don thought. He grabbed it with his teeth. while she was focused on Luke. Then Don creeped up behind her and CRASH. “What!? You son of a-” She didn’t finish her sentence as she started to feel drowsy.
Luke watched in surprise as she wavered back and forth, trying to keep her balance. She looked at Don, a burning fury in her eyes. “You. You were the one I loved. No more hope for you. For either of you! You’ll change, and in those forms you’ll stay! For eternity!”
She ushered out those words before she collapsed, snoring heavily. Luke looked at Don, who was officially taller than him. Don’s cat ears were laid back against his head, his hopes dashed.
They ran up the stairs and through the house. Don stopped for a minute as his waist began to get thinner. He fell as his hip realigned themselves. He yowled as the sounds of bones crushing and insides squashing filled his ears. His feet began changing too. His big toe moving farther up his leg. He examined his foot seeing pink paw pads form on each digit. He let out another yowl as he dropped his foot and landed forward on his front paws. His tail stiffened as a sharp crack echoed. Luke ran to him, his height dropped by another couple of inches, as he tried to help him up. He could feel the fur creeping up his stomach, but he chose to ignore it as he tried to assist his friend in need.
They got to the door and realized that it was too high up. They jumped but couldn’t reach the knob. Luke got on Don and grabbed the knob. Luke felt his face began to push out. He squeaked as his face shifted into what was a rat muzzle. Whiskers protruded out and his new pink nose wiggled. His skull receded as his eyes moved to the side of his head. While normally gray, they were washed over, becoming black and beady. More gray fur washed over his face, helping his previously changed ears match. His front teeth protruded out, becoming longer and more rodent-like. His height was dropping drastically, and he barely managed to open the door before he couldn’t reach it any longer.
The fur flooded up his arms, the gray fur stopping at his hands as they, too, became pink and clawed. He examined them for a moment, a little in shock, before he brought his attention back to Don and the open door.
They ran out the doorm fully transformed. They ran and ran until they couldn’t breathe. Don meowed. Is it over. Are we done? Luke squeaked. Then a sound filled Don’s ears. The sound of footsteps.
Luke looked up in panic. He was so small, everything was so large. He felt overwhelmed and the courage he had before was gone.
“A cat?”
“And… A mouse?”
“That’s a rat, dweeb!”
“How can you tell?”
“I just can!”
The two kids walked up to them. The girl had brown hair and looked about 7 and wore a green dress. The boy was older. He had to be at least 13. He had crazy blond hair that stood up in all different directions. Luke squeaked to Don, Are they going to hurt us?
Don replied back with a mew, No. I don’t think so.
The little girl walked up to Don and picked him up. “He’s cute. Think mom will let us keep him?”
The boy looked the cat over and frowned. “I don’t see a collar. So, yeah, I don’t see why not. She’s great about letting us take in needy animals.”
“Then let’s go take this cat home!” They began to walk, but Don began to yowl, reaching for Luke, who squeaked.
“The rat? He wants the rat?”
“Probably to eat it.”
“No, I don’t think so!” The girl put the cat down and watched as he walked to the rat.
Don trotted up to Luke and bent his head down. Need a lift? Don asked, laughing. Luke hesitated, but he hopped on Don’s head and Don trotted back to the girl. The boy rolled his eyes “Great, now we have another problem.”
“Shut up.” She yelled at him. “I think it’s really cute!”
“Alright, alright. Mom’s gonna hate thiiiiis, though!”
“She’ll understand! She’s awesome!”
The boy bent down and carefully picked up Luke. “I bet it’s got all sorts of diseases.”
“You’re so mean! I bet it’s not!”
“We’ll see! Let’s take them home.”
Don’s throat began to rumble. It was a little strange at first but he relaxed in the girl's arms.
“Oh no, he’s going to explode!” The brother teased her.
She ignored him and began to pet the new cat. “We’re going to take good care of you and your friend.”
Together, they headed home with their new pets.