Hello guys! Please enjoy this little AU. It all stems from a simple question: What if Sans had fallen into the Core instead of Gaster?

I'm working on a much bigger piece involving Dr. Gaster (to be published at a later date), and so this ended up being a bit of a warm up to that. Obviously I had to make some assumptions about the guy, so I hope you like how I've written him. You'll notice Papyrus is a bit out of character as well, funny how removing a major character will do that. But Papyrus still has his heart, not all of who is is relies on his big brother.

Not all is as it seems in this story. I would love to know what you think and theorize along the way!

Thank you for reading!

Oh and Disclaimer: I don't own any of these characters, or Undertale. Go figure.

Every night was the same. Gaster woke in a panic, grasping at air and struggling for breath. Violet light flickered in the dark room, pulsing through him with panicked speed. He was frozen, a shuddering arm extended and reaching for a hand that he would never catch. After many long moments, his magic slowed it's throbbing, stilling to a hum that echoed years of grief. Slowly he lowered his tense limb, sucking in air to calm himself until the magic light dimmed. Haunting darkness took over.

Gaster slid his legs up until his knees touched his chest, holding them in place with wrapped arms. The action was a childish instinct, and he convinced himself he wasn't yet lucid enough to control or be embarrassed by it.

The nightmare began to fade away, and he almost panicked again. He reluctantly tried to cling to the images, registering them to memory. However unpleasant, Gaster wanted to remember. It was all he had left of his son.

He went over every detail, recalling the boy's hands reaching out for him, his small body as it was pulled away, and his face. His child's persisting smile turned to a terrified grimace as darkness overtook him, the light of his soul disappearing like a candle's flame that had been snuffed out.

The core had been their greatest project together. It powered the whole of the underground, making life for monsters more bearable, happier. But there was so much power. They worked tirelessly to keep it stable, doing test after test to find some permanent way to contain the raw energy within. Both of them were affected by it for how closely and often they worked, the properties of their magic changing together. It was as worrying as it was intriguing. If the core could manipulate their magic, then maybe they could use it to manipulate the barrier trapping them.

They should not have kept going...

The Core held immense power they did not understand. Could not understand, Gaster realized, far too late. The magic within it had the potential to affect far more than they expected. Instead of destroying the barrier, the seemed to connect to it, and then turn on them. It reacted violently. Fearing all would be lost, the scientists did everything in their power to stop the inevitable. They had feared for the lives of every monster, and in the end, only they were ever affected. His small team was taken...and so was Sans.

Gaster stretched out again with a miserable sigh. He clenched his fingers, grabbing up blankets in between them.

There was no dust left of them. Nothing to show their families or his other child. They were all simply gone. Sans was gone.

Time passed, and other things disappeared. Old pictures, his books and clothes. At first Gaster thought Papyrus was taking and hiding them. When he questioned his youngest son he was met with a more terrifying revelation.

"Who's Sans?" The boy had asked. "Isn't that someone you work with?"

Gaster felt he had lost all hope when everything belonging to Sans vanished from their home. Before long he realized the same was happening at the homes of the other scientists that disappeared into the Core. Their families had no memories of them.

Papyrus, who was so close to his brother before, struggled with diminishing memories of his brother. Gaster returned to their lab again in despair.

He found renewed hope there. Everything of his son's was still there. Files, notes, even the leftovers of a lunch they'd shared still remained. Sans's favorite blue jacket still hung from a hook on the wall. Everything that was still close to the core. The scientist, after much deliberation, decided to bring Papyrus to the lab. He was scared it wouldn't be safe, but he had a theory to test. Gaster was careful, unwilling to lose a second son. Though Papyrus's memories didn't return, what he had left over was strengthened upon entering the lab. He remembered his brother, and clung to every bit of him he could.

After that, Papyrus's memory fluctuated. He never seemed to completely forget, but some days he seemed to more than others. It unsettled them both, but Papyrus at least could be soothed by stories and memories Gaster shared with him. Both skeletons were too scared to take anything away from the lab, lest the few remaining items disappear. Time went on. They were the only ones who knew of and remembered Sans. Gaster frequently dreamt of the day they'd lost him…

A sharp knock at his door startled him out of his thoughts, making him jump.

"Dad? Are you awake?"

Gaster sighed again, a short breath of relief. "Yes Papyrus, come in."

The door creaked open, and the boy stepped in. He was hunched over a little, clearly tired, but he flashed a shaky smile. Gaster made an attempt to return it.

"Did I wake you?" he asked. Papyrus shook his head.

"I did not sleep very much. I thought I would leave early for my patrol."

Papyrus was already dressed to leave. Checking the time, Gaster realized there was perhaps another hour or so before they both normally woke up. He swung his legs over the side of his bed. "I suppose I may as well stay up myself," he said quietly.

His son nodded, understanding. "I made breakfast already. There uh...might be a little extra."

Gaster waved a hand, a true and knowing smile lighting up his features. "I'll take it to the lab with me," he suggested. "Alphys could do with something other than instant noodles for once."

Papyrus brightened, standing up straight. His full height was now very near that of Gaster's own, and for a moment, the father marveled at the time that had passed to allow such growth. Looking at his son, he felt truly grateful to have him still.

Gaster stood up slowly, headed to his closet. "Do you have err, training with Undyne today?"

"Yes!" Papyrus grinned widely. "The Great Papyrus must keep on his toes after all!"

The father chuckled warmly. He continued to get ready, listening to the younger skeleton chatter about his plans and excitement over becoming one of the Royal Guard, the stress from his nightmare fading away.

Papyrus was what kept him going through the nightmares. When Gaster thought he had lost all hope, his youngest son reminded him of what he had yet. The boy also quieted perhaps his greatest fear...

"Dad? B-brother would have been proud of me right?"

Gaster closed the door behind them, taking his time with the lock. It was still and quiet outside, being so early yet. Snowdin had not been his ideal choice for new home, but the small town was peaceful, and easily grew on him over the years.

In the stillness, the click of the lock sliding into place seemed to echo a bit louder than normal, and Gaster took a moment before facing his son. He had no doubt in his answer, but somehow it was still difficult to say.

"He would be, " He said, forcing feeling into his voice. "Sans would be so proud of you."

Papyrus did indeed remember Sans, but their relationship was something he still asked his father about. Sans had always doted on his younger brother, doing everything in hopes to keep him happy and healthy. Gaster was certain that Sans held no one else in a higher light than Papyrus .

"Sans loved you," he made sure to catch his son's eye.

Papyrus nodded, his smile only revealing a hint of sadness. "He loved me," he repeated. "Right."

Gaster put a hand on the younger skeleton's shoulder, squeezing it. The gesture offered comfort, and acted as an invitation that Papyrus often accepted. The boy stepped in, wrapping his arms around Gaster in a quick and meaningful hug, then they went their separate ways.

While he enjoyed the peacefulness of the town, Gaster still didn't care for the biting cold wind that blew through on occasion. Still, he was content to walk slower than normal this morning on his way to where the River Person docked. They were already there waiting. Gaster assumed normally that he was their first passenger most days. He never seemed to have to wait on the mysterious monster. He climbed onto the ferry, simply nodding to it's owner, a signal his destination was the same as always.

He rarely paid attention to the passing sights on these trips, and this time was no different. Gaster's mind had settled back into gloom, the sing-song of the River Person tuned out. He thought again on his dream, straining to remember the earlier images, where Sans was still whole, and terror didn't yet show on his face.

What he was able to recall anymore was blurred and incomplete. While he had allowed the dream to slip from his mind for a short while, it had gone too far. To forget completely, he felt, would be a betrayal to his eldest son. He stamped out whatever tiny relief he had from the nightmare receding to a fuzzy picture.

When the ferry stopped, Gaster walked a bit faster to the lab, stressed and eager to dive into any work to keep his buzzing mind distracted.

After the core incident, Gaster worked alone for some time. He searched for anyway he could think of to try and return Sans and his old team to this world. Numerous machines were built and countless tests run to find some hope that he could bring them back. Little progress was made, and after several years yielding no results, Gaster pushed aside his blueprints and notes. Feeling as if he were turning his back on those that disappeared, but unable to cope an longer with the false hope, he concluded his team and his son were gone for good.

He couldn't get rid of all the work he'd done though. It was tucked away in the lab, safe, and accessible.

He turned his focus to other things to help monsters in the underground, working alone until Alphys impressed with her mechanical prowess, and understanding of soul energy and magic. The king strongly suggested Gaster take her under his wing and he did. It was an awkward arrangement at first, but Alphys proved herself, and soon enough Gaster became pleased with the company, despite her tendency to over-talk.

The lizard girl practically moved into the lab itself for how much she was there. She had a sleep schedule that even after their now several years of working together, Gaster still hadn't found any pattern in. But try as he might to instill a more healthy regime (and diet) for her, Alphys kept to her own habits.

Gaster entered the building, still moving briskly, keen on settling himself into the first of many waiting projects that met his eye. Not surprisingly, Alphys was standing in the main lab area when he walked in. He muttered a good morning and dropped the bundle of food from Papyrus on the table behind her in the center of the room. He went straight to his own desk and lifted a stack of notes he'd left from the night before, all the while mentally preparing himself for a wordy synopsis of whatever show his coworker had watched while he was gone.

But Alphys had not moved. After too long a silence, Gaster slowly looked away from his papers. Alphys was staring, transfixed, at the large monitor against the wall. It was an old security system from before the incident that linked to several cameras around the underground to be on lookout for humans that fell. Alphys had made it her own project to fix up the outdated system. She added in an alarm system that activated the cameras automatically when someone was nearby, alerting them at the same time. It was more efficient, saving power, time and stress. Her efforts with the system was impressive, to say the least. The older scientist had held no patience for it.

Gaster watched her. Her eyes moved swiftly across the screen and every once in a while she would look down to enter a command into the computer then dart her eyes back up, looking panicked. The cameras were on.

"Dr. Alphys?" he called. She jumped.

"D-Dr. Gaster!" she squeaked, turning slightly to look at him. She was unwilling to turn her attention completely away from the screen however, her eyes darting back and forth between him and it. "I'm s-s-sorry! I d-didn't hear you c-come in!"

"It's quite alright," Gaster assured. He gestured at the screen. "Has something happened?"

"Oh, y-yes!" Alphys squeaked again. "I was uh, l-looking into some of our old notes on that anomaly we were looking into last night. J-just to see if I could get any new ideas s-since we haven't been able to detect anything new in a l-long time. T-then after reading those for hours I wondered about our old theory that maybe it was someone in particular causing the strange j-jumps? S-so I turned on the cameras and s-started flipping through the channels to see if I could find anything out of the ordinary. A-and w-well…"

She turned fully to the screen again, and typed in a command to change the camera feed. Gaster stepped closer to see. Someone was moving across the screen, but he couldn't quite make out who. He could tell this feed was from Snowdin however, or just outside of the small town he'd just come from.

"What is this Alphys?" he asked, squinting at the screen. His eyes, unfortunately, were not as good as they once were.

"Shortly after I t-turned the cameras on, the alarm I set up went off. It took me a bit to f-find the right camera they set off b-but...I've seen them on n-numerous feeds now," Alphys shook, staring at the screen anxiously. "They m-must have fallen late last night or this m-morning, but they only j-just exited the Ruins," she swallowed, wringing her hands. "It...it's a human."

The papers dropped from Gaster's hands, hitting the floor as a pack before scattering across the floor and his feet. In shock, he gaped at the screen with her. As they watched, he recognized where they were at. It made sense, all the humans before had fallen in the Ruins. There was only one direction from there. The human, only a child it seemed, was walking down the path towards Snowdin from the old ruins. He knew he needed to raise the alarm and send the royal guard after the human, but something else had him frozen in place, watching as the child stopped in their path.

Standing ahead of them, blocking the way, was Papyrus.