Nope, it was definitely a tail.

It looked like his tailbone had grown into an actual tail, extending down to the length of his knees. The tail was made out of bones that resembled his vertebrae bones, as if it was an extension of his vertebrae. Sans tested it out by trying to move his tail, finding that it flicked intentionally. Yup, he had a tail now. It felt unusual, like he suddenly grew another limb, which wasn't very far from the reality of the situation. The tail moved when he wanted it to, bouncing up before falling back onto the chair. And Sans felt the impact, too. Definitely his tail.

Chara was giggling into her hand, snorting lightly. Sans carefully picked up his newly formed tail and examined it, feeling the bones under his hands. It didn't feel like magic, where Sans could sense it and desummon it. No, it felt like an actual extension of his body, like it was just another part of him he's always had.

"I was thinking you were going to get some horns first, but... pft, even the tail is short," Chara chuckled, not bothering to hide her glee.

"Wow, thanks," Sans said sarcastically, pulling at the tail slightly. "Yup, it's definitely connected to me. Now I got some random tail."

After the tail had formed, he immediately felt less sick. Likely due to the fact that the soul restraint was broken down enough to allow Sans to have a tail, so his magic wasn't as strained anymore. It wasn't gone, Sans still felt like crap, but it was a little better.

"At least it shows that you're getting progress," Chara said, "It's not demon powers, but it's a step. I'm not really sure how I'd train your tail, though."

"Will it go away when the blood wears off?" Sans questioned, poking the new bones. "It doesn't feel like magic, like it feels really real."

"I'm not really sure, I'm not an expert in half demon training," Chara muttered, "But if I had to guess, I'd say it'll probably stay. You should have been born with some demon traits, really, but it looks like the restriction stopped them from forming until now. I also know that Zalgo has some files-"

"That's where they went!"

Sans took a deep breath, finally finding the solution to his problem that had been bugging him for years. Someone had taken a few files on determination written by Gaster himself, and it had worried the skeleton to no end. He actually thought Alphys was the one who took it, and that she had some plans to use determination to stop the CreepyPasta. At least that wasn't the case!

But... then why would Zalgo need those files? What were his plans? When a single relief met Sans, another question bubbled up within his mind. He was already very aware that Zalgo was planning something, this just brought more worry to the table.

"Huh?" Chara asked.

"I knew that the files had disappeared, I just didn't know who took them. At least that answers my question," Sans explained, "Do you know why he needs it?"

"No, he doesn't let me know that kind of stuff. He just tells me what to do, I don't really know why he even wanted to bring you down here in the first place."

"Oh, that's just great."

If it was easy to read Zalgo, then maybe Sans would have a clue on what the man wanted. But Zalgo wasn't easy to read, quite the opposite. The demon was good, extremely good. Sans could barely predict was he was thinking, his mood often changed so quickly that it was even hard to pinpoint what he would do next. It didn't help that Zalgo seemed both head over heels for his family while being secretive at the same time, so Sans wasn't sure if he was faking the family part because he needed them or not. All Sans knew was that Zalgo needed them all off of the surface for a reason, and that was it. Or that he at least wanted them off the surface in hell, Sans couldn't think of any good reasons to why Zalgo needed Sans and his family.

And speaking of Zalgo, the demon seemed to have impeccable timing as he walked into the room at that very moment.

"There you are!" Zalgo said, grinning at his supposed son. "Your bedroom was empty so... I..."

The red eyes glanced down at the seat, noticing Sans' tail as it shifted on the wood. Every mouth on his body pulled back into a larger grin, his own tail perking up.

"Right, uh, so I got this now," Sans said, gesturing towards the tail. "Chara and I-"

"Oh myself, that is so f*cking CUTE!" Zalgo roared, knocking aside an entire table before he was standing next to Sans in almost an instant. "Look at your tiny tail!"

"It's not that tiny! Why does everyone keep saying that?" Sans asked, before he noticed Zalgo's tail. "Are you wagging your tail?"

Before he could object, Zalgo was grabbing Sans' tail, wiggling it within his hand. "It's so short and adorable! You're like a baby demon!"

"Please stop touching my tail," Sans said, trying and failing to pull his tail away from the demon lord.

"And I bet your horns are going to be short too, aw!" Zalgo practically cooed, pulling at his tail again. "It's just so short! Look at it, it's just so short and pointy!"

"I know! It's so much shorter than all of the other demons tails," Chara agreed, grinning at the demon.

"He won't be intimidating in battle, but I don't care because of how cute my son is!" Zalgo practically roared, drawing Sans into a giant bear hug. "You are going to kick so much a** while also looking like my little baby!"

"Help me!" Sans said, wiggling his arms at Chara.

"I think Sans wants you to hug him tighter, he really needs some hugs from his Papa," Chara teased, shrugging when Sans gave her a death look.

"You have to meet your sister now! You two would be the ultimate cute child duo!" Zalgo said, picking up Sans. "We're going to go meet her now!"