Author's Note: Hey guys! My name's LittleTeenAuthor or just plain Soph. I welcome you to Undercross! A story about the Multiverse of Undertale and basically how Sans needs to save the Classic Universe with the help of the other AU Sanses(Plural?). Yes, this is the first part of a series, probably about 4 or 5 stories long. Anyhow! Welcome... To Undercross...

Disclaimer: I do not own anything here except for the plot! Undertale belongs to Toby Fox and all the little skelies belong to their respectful owners!

Chapter 1: Why Mondays?

Sans hates Mondays. Especially the ones where he has to wake up to babysit the kid. Honestly, he doesn't mind being with the human but waking up is annoying. After the kid promised to not reset anymore, Sans has tried to stop being lazy but sometimes you just can't get rid of old habits.

He walked in silence from Snowdin to New Home, letting his thoughts run like a river. Well, that's usually what happens anyways. Today, however, his skull is empty and he doesn't like that. He may be lazy but he's a deep thinker and having a cleared head is strange to him. It was giving him a bad feeling.

After some time walking, he finally reached his destination. Why didn't I just teleport? he thought as he looked up at the big door. Then he remembered the promise he made to himself, Oh yeah. Welp, guess I better head in. Toriel would chew me out if I was late. He knocked on the door, "Knock, knock."

"Who's there?" answered a soft but fierce voice from behind the door. It was like a mother, soft but fierce.

"Orange."

"Orange who?"

"Orange you glad I'm not late?" Sans chuckled to his own pun.

The door opened to reveal a female goat monster hollering and snorting crazily. "Oh, Sans. That one was good." She managed to break from laughing to speak.

Sans grinned nervously, "Yeah. Wassup, Tori?"

"Oh, I'm fine. Frisk has been waiting you know. She wouldn't come out of her room all morning." Toriel looked worried which made Sans be worried but he hid it.

"Welp, guess you can say that Frisk is so bonely in her room all abone." His puns gained a couple of snorts from Toriel. He chuckled a little too but whenever he's with Toriel it's always so… awkward. He rubbed his neck a little, "Uh, so don't you need to meet at the Assembly today?"

Toriel's happy face turned into realization, "Oh, yes! Sans, please do take care of Frisk. I'm worried for her, she rarely comes out anymore. Only to eat and when we have guests."

Sans gave Tori a "thumbs-up" while she left New Home to the barrier. When Frisk broke the barrier, monsters flooded to the nearest city to quickly look but the Royal Family decided for monsters to stay Underground because they didn't want to crowd the city too much. So now monsters and humans can travel at any time to each other's territories. Most monsters preferred to be Underground but some ambitious ones decided to explore the surface and travel the world. Those monsters became known as the "New World Explorers." Frisk wants to be one when she's old enough.

Sans walked in inside and shut the door behind. He walked to Frisk's room and called out to her, "Hey, kid, ya there?" When he got no answer, he was about to open the door when he heard voices coming from inside. This instantly made his left eye flare up in a blue flame and barge through. He readied his blasters to appear on a moment's notice.

The kid was sitting on the floor with two other skeletons sitting around her. Sans stopped and his eye dimmed to normal. He was confused. The inhabitants of the room all noticed him.

The two skeletons looked like him but not like him. One had a black hoodie, red pupils, black shorts, black and white sneakers, and a red undershirt. The other had a blue scarf, light gray shirt, dark gray pants, and blue boots. His eyes were blue too and had a star in each. Frisk had her normal attire: blue long-sleeved shirt with magenta stripes, blue shorts, and brown shoes.

Sans stumbled back. "Uh, ok. This is weird." he said to himself.

The skeleton with a black hoodie stood up and spoke, "Right, who the flipping h-" He quickly corrected himself while glancing at the other skeleton next to him, "Sorry, seagull, are you?" He pointed his finger at Sans.

"Sans. Sans the skeleton. And, uh, same to you, pal." Sans dropped his confusion and replaced it with seriousness. He stood up straight to look taller than the other skeleton speaking to him.

"Name's Fell, 'pal.'" Fell folded his arms onto his chest and laid back on the wall behind him. Sans noticed Fell had a gold tooth too.

"'Fell,' huh? Is it because your clothes look like they fell into something really dirty?" If Fell wants to be rude, Sans will return the favor.

Fell growled, "Why you little-"

Frisk stood up and interrupted him, "Sans, these are my friends. I guess you met Fell already." She motioned to Fell and then to the other skeleton that was still on the floor. "And this is Blueberry." She introduced Blueberry with a smile. Blueberry waves happily and Sans half waved back.

"Hey, kid, can I talk to you? Outside?" asked Sans. Frisk nodded and followed him out the room into the gray hallway.

"Right, explain why those two look like me?" Sans went straight to the point.

"They're from different versions of you!" Frisk said that too happily and started jumping that Sans had to calm her down. "Sheesh, kid, calm down. So they're from a different dimension?" The kid nodded. "Ok, welp, that explains some things." He looked down for a moment. "But how'd they get here, kid?"

Frisk happiness disappeared instantly, "Um… Well… I don't think I should… Um… Actually, I think it would be better for me to tell you." She mumbled, "They'll probably close up again."

"Tell me what?" Sans was filled with worry, not only for the kid but for himself. Things usually all go downhill when the kid is secretive. Honestly, Sans has a lot of jobs. Being secretive is pretty high on the list.

"They… They lost their universes. Both of them found each other and dimension hopped for a while until they ended up here. The 'Classic Universe.'" Frisk explained. Her voice was sad and low. Her lips were twitching. Oh geez, please don't cry, Frisk, thought Sans.

"Apparently, we're part of an entire multiverse. Like in those human superhero comics!" Frisk instantly brightened up a little and jumped on Sans before he could react. He yelped and fell. Luckily, it wasn't a fall from a high place, like off a table, or else he could've died.

Frisk was giggling and Sans laughed a little too, "Heh. Alright, kiddo, time to get up. We don't want to keep your 'friends' waiting." Frisk nodded and headed into her room. Right before she went in though, Sans noticed a small amount of glitch in the hand holding the door open. It reminded Sans of someone else but he shook off the thought. Right now, he needs a reintroduction to Fell and Blue.

A Few Weeks Before…

Ink struggled against the strings but it all seemed in vain. He never would've thought that his brother would've done this. It's always just a small battle in most dimensions. In others, it could just be none. That is, when Error finds the universe pathetic. But now? Now Ink's trapped in Error's blue strings in the anti-void, his own home.

His paintbrush is right there, just a couple of feet away. If only he could grab it, then he could escape and stop Error. Heck, Ink can't even open a portal or reach one of his paint vials. He still struggled against the strings. When has Error ever been this strong? Seems like Ink underestimated him.

The strings hold tight that's for sure. Ink is the Guardian of the Multiverse; he's supposed to be as strong as Error. Well, he did put up a good fight but he can't bring himself to really hurt Error. And now he's stuck. Which is just great. Stuck in midair so it's even better.

"Oh please, you'll never break them, ya know?"

Ink stopped what he was doing and looked down to the white "floor." A black skeleton stood looking up right back at Ink. Error was smiling up at Ink, probably in victory. Then he cocked his head to the side as if listening to someone. After a couple of seconds, his attention returned to Ink. "The voices really don't want me to kill you." Then he gave an evil grin. "Shame." He laughed a little while Ink gave a low growl.

Error picked up on the growl and a red bone appeared under him and lifted him straight up to Ink's level. "Hey, don't be upset. We had a killer battle, didn't we? You're lucky the voices are persuasive." He summoned several threads of string in his hand, "But they didn't say anything about hurting you." He sent the strings to Ink's soul and yanked it out. Ink winced but when the strings started to squeeze the soul, he couldn't help but scream in pain. Error grinned as Ink screamed and yanked the soul even harder. Finally, he let go of the soul and let it return to Ink's body. The screams stopped and Ink was panting heavily.

"Is that… Is that all you got?" Ink said in between pants, "You… You should've put more backbone into it…" He smiled at Error's annoyed face.

"You realize that isn't funny right?" Error said. "You're literally asking for more pain." He cocked his head again and started talking to no one in particular, "Of course I like puns! Except, I literally just tortured him and he's laughing it off." His attention returned to Ink once again when he realized his opponent was confused. "The voices. They can be so annoying sometimes."

"What universe are you destroying this time, Error?" Ink asks. He's recovered a little now.

"You seriously think I'll tell you? Geez, you really are dumb." responded Error.

"But let's just say that you're gonna be unemployed when I complete my plan." He smiled mischievously at Ink. "Welp, ta-ta! I have a universe to destroy, Ink, and I prefer to do it now." The red bone lowered down quickly and Error walked a couple of meters before opening a portal. "Oh, and Ink? I hope you like silence." He grinned evilly and walked through his portal, leaving Ink alone in the silent, vast, white void. He had a feeling he'll be here for a long time.

Frisk was playing tag with Blueberry while Sans and Fell chatted away. They sat in the dining table, watching the other two have fun. Sans apologized for his rudeness and Fell barely managed to do the same.

"So… Underfell, huh?" asked Sans. "What's it like there?"

Fell shrugged, "Pretty horrible. Especially when you have to live with a pretty messed up bro. Though, the human hasn't changed for both of our universes." Sans nodded and continued with the questions. "I'mma say that you're not very fond of your Paps?"

"Oh jeez, what made you think of that?" Fell answered rolling his eyes, which gained him a blank stare from Sans. "Sorry, it's a habit." he answered. It wasn't a full apology but Sans let it slide. He could tell that the other skeleton really only tolerates him.

"How'd ya get here, Fell?"

Fell seem to flinch at the question, something Sans hadn't seen from him until now. The red-eyed skeleton looked down, putting his mouth in his hoodie, and he mumbled unintelligently. Sans continued to stare at the skeleton until Fell realized what Sans wanted. "My universe is getting destroyed."

Of course, the human already told Sans that but it still was frightening enough to remind him of Chara. Of Chara and what she and the kid almost did. Good thing Frisk aborted that path. Still though, you don't take "my universe is getting destroyed" lightly. Ever. It stuck Sans like Chara's knife. It still struck Sans like Chara's knife.

Sans was pretty sure Fell could tell what Sans wanted to know without asking it.

"Yeah. It was a pretty normal day for me. Papyrus chewing me out like the idiot he is, random monsters staring at me, wanting to dust me. Yep. Until I noticed these like 'Errors' and pieces of code missing in my house. Found this portal thing and went through. I ended up with the stupid little blueberry 'cause of it." explained Fell. Sans could tell it was a touchy subject for him so he pretty much regretted asking it but Fell wasn't done. "Heh, turns out we both ended up in the kid's room." Fell rubbed the back of his neck. "And apparently your Tori is nice? Not really something I see at my place. She tried to bake me into a pie once. It was horrible." Fell shuddered at the memory.

Sans looked down and over to where the kid was supposed to be playing. She wasn't there. This instantly alarmed Sans and his magic eye threatened to flare up again.

He felt something start crawling on him and he turned his skull to behind him to see the kid climbing all over the two of them. He turned to Fell and he looked pretty annoyed by it. He could see the struggle Fell had to deal with as to not curse in front of the kid or hit her. Sans laughed and turned Frisk's soul blue. He lifted the kid up and put her on the floor, where she fell with a flop and giggled.

Blue came out of the kitchen, "Hey friends, I made tacos!" Everyone turned to him.

"How'd the literal heck can you make tacos so dang fast?" asked Fell.

"Maaaaagiiiiiiiic." answered Blue and popped back into the kitchen to grab the taco tray and brought them out. Everyone ate and Sans was pretty surprised at how good they were, considering that his cooking skills are limited to ordering pizza and making small snacks, like sandwiches and hot dogs. Fell ate in silence, Frisk was chatting away with Blue, something about the Royal Guard, and Sans' thoughts returned.

"Hey Blue, where ya from?" asked Sans. He finished eating the last of his tacos.

Blue turned to Sans and gulped down some taco piece he was munching on, "Oh, I'm from Underswap!"

"…Underswap?" Sans gulfed down some ketchup to wash away the hard taco shell. Another habit he hasn't gotten rid of, and frankly, probably would never.

"Yeah! My Paps is a lazybones and I am a future Royal Guardsman." Blue stood up on his chair and posed heroically as if there was wind blowing against him. "Mweh heh heh heh!" Frisk giggled but it soon turned into a joyful laugh. Fell groaned and looked like he was so done with the little blueberry and Sans chuckled with the kid.

Suddenly, the door slammed open. But nobody came.

A/N: Hey guys, it's me again! Wow... Some cliffhanger, huh? *evil laugh*

Damien: Uh... Soph?

Soph: What?

Damien: Nevermind *walks out*

Disclaimer: I do not own anything here except for the plot! Undertale belongs to Toby Fox and all the little skelies belong to their respectful owners!

EDIT: Fixed some grammar and made Fell more Fell.