A/N: So I normally don't attempt multichapter fics, but I really want to try to write this. Hopefully I'll be able to finish it! Let me know what you think! By the way, italics means he's speaking in wing dings, since I can't use the actual font. Enjoy!
The subject of how monsters are created is one that is, surprisingly, not discussed often. It was widely accepted that monsters just showed up on the Surface one day and continued multiplying from there. No one could hold anything against the humans for being afraid. After all, they had been there longer, long before any monster stepped foot on the Surface, and to be scared of things that were new or not understood was simply human nature. Monsters believed that, eventually, the humans would get used to the sight of them and they could live together, peacefully, as they had been for years after the monsters appeared. No one had expected the war, and certainly no one had expected the humans to be so strong against the monsters. A child could kill a monster, even effortlessly if they had the right intent.
In the end, it could hardly be called a war.
The few monsters that were left standing at the end were forced back into a cave leading into the depths of the Underground, forced to watch helplessly as seven humans cast a magic spell over the entrance to the cave, sealing it with a barrier that no monster could ever hope to break. It would take thousands more monsters than those who were left to equal the power of one human soul, let alone seven. King Asgore and Queen Toriel stood side by side at the head of the group, Asgore watching with a sense of hopeless resignation as Toriel turned away from her husband and quickly began comforting the monsters, tending to any with wounds as best she could.
Towards the back of the group stood the only one who refused to accept the fate of his race. His career choice required him to occasionally accept failure, accept that some things just couldn't be done, but he absolutely refused to accept this. Above all else, Gaster was a scientist, and he wouldn't accept defeat until it stood unopposed before him. He studied the Barrier, taking mental notes as he began running through ideas on how to break it. Human souls were terrifyingly powerful, their magic something that no ordinary monster could reverse. Monsters could absorb one human soul, but even that much made them unstable. Attempting to absorb seven would surely turn a monster to dust. His thoughts were interrupted by a familiar hand landing on his shoulder. Gaster looked up and saw Asgore examining his expression with a strained smile.
"I know that look, Gaster," he rumbled. "What're you thinking?" Gaster turned his gaze back to the Barrier, the light emanating from it blocking out everything else in the darkness of the cave they were still huddled in.
"I'm thinking…" Gaster replied slowly, "that we have to do something about that." He gestured to the Barrier, eyes narrowing as he noticed it seemed to be pulsing. The humans had used the power of seven souls to create it, making a huge energy output. The pulsing could be a sign of instability from using too much magic all at once. If that were the case, maybe the raw power of seven more souls would be enough to break the barrier and no one would need to absorb them. Either way, they'd need human souls, and with the Barrier shutting off monsters from the humans, the odds of the two races ever meeting again was slim.
"Ever the scientist, aren't you, Gaster?" Asgore said with a weak chuckle, trying to lighten the mood. Gaster offered him a small smile.
"You know me, Asgore."
Yes, ever the scientist, indeed. Because he could still smell the fresh air of the Surface, sweet and promising, and it only made the stale air down here all the more suffocating. The idea of their race having to spend the rest of eternity trapped down here was choking and made the all too small walls close in even further. Gaster saw Asgore also turn to watch the throbbing light of the Barrier as his hand slowly fell from the shorter man's shoulder.
"I have no doubt you'll get us out, Gaster. But let's at least get settled first. I have a feeling we'll be here a while." Gaster didn't argue. There wasn't much he could do now, anyway, and they had just fought a war. Forming colonizations and settling down for a little while was a necessity, although he wasn't exactly thrilled about it. Monsters all agreed to it when Asgore announced that as their plan, and not one questioned how they were going to get back to the Surface. They'd been down here for maybe an hour and already monsters were resigning themselves to living here, forever. They began filing out of the cave to explore the rest of the Underground, Toriel and Asgore taking the lead. Gaster was the last to leave, getting one last glimpse of the Barrier before following everyone.
He had to break that Barrier fast, while there were still hopefully a few monsters with the motivation to help him do so. He was a brilliant mind, but accomplishing the task alone would be impossible.
'Settling down' took much longer than Gaster had anticipated. There was a surprising amount of land to the Underground. As they went, they found other edges to the Barrier, likely to keep them from accessing the Surface through another cave. Three civilizations developed, monsters spreading out to the space based on whichever area better suited their needs. The king and queen, once all if the Underground had been thoroughly combed, returned to the spot where they'd all initially been forced Underground. A thriving boom town quickly formed around the castle they built there, the area being labeled 'New Home.'
Asgore never had been very good with names, whcih showed itself in the names of the other main areas of the Underground. The edge of the Barrier directly opposite Home was the Ruins, though not many monsters went there. It was empty and abandoned, and filled with spikes and other such traps that most had no desire to live around. After the Ruins was a frozen tundra, perpetually cold and snowing. Despite the constant flakes that drizzled down from the sky, it wasn't too cold, at least not by monsters' standards, and a town much more homey than the desolate halls of the Ruins formed, dubbed Snowdin. After Snowdin was a maze of caves and corridors, filled with rushing water and a ceiling that glittered with glowing stones, reminiscent of the stars that shone in the sky over the Surface. The area was called Waterfall, and it briefly caught Gaster's interest as it seemed to have a direct link from the Surface that wasn't blocked off by an edge of the Barrier. Monsters couldn't use it to get out, but it seemed to provide them with trash and junk from the humans. It gave everyone, Gaster included, hope that there were maybe more 'holes' in the Barrier, and that maybe one of these 'holes' would allow them to get back to the Surface without ever needing the human souls.
However, the area that really drew in Gaster, and the area that he himself chose to settle into, was the Hotlands. Contrasting sharply with Snowdin and Waterfall, the Hotlands cut through the earth's crust, creating pools of magma and a heat that would be unbearable to any human. It was the perfect area for Gaster to begin executing his plans, especially since Asgore had given him the official title of Royal Scientist. The heat here, as well as the direct access to the magma, would allow Gaster to use geothermal energy entirely to power the Underground. After all, they couldn't use the humans' electricity anymore, and they definitely couldn't use solar energy. The sun was long gone, and a lot more monsters had died off in the years following the formation of the Barrier, some races that depended on the sun even going extinct.
But that didn't matter now. He had work to do, a science division to rebuild, and a CORE to make. In the end, it would do so much more than convert geothermal energy into magical electricity, but no one needed to know about the duality of the CORE's use.
Developing the CORE, as well as developing a well-rounded team that could build it and help him with future projects took a while, and the entire process was carefully observed by him. Some parts of the CORE's development Gaster had to handle personally, as there were a few extras he had to add in order for it to work, but it would all be worth it. The CORE was his magnum opus, and while it wasn't the only thing he was researching—the humans' mysterious SAVE and RESET abilities that they'd utilized in the war has certainly been interesting—it was definitely his priority. He worked long hours, staying holed up in the lab long after everyone else had left in order to work out some error or to improve the design somehow. He'd had to scramble to prevent the damage one specific error could create, one that could allow the entire system to overheat—he'd had a long talk with the engineers, because he apparently had to explain that a machine working twenty-four/seven with magma would require a central cooling system. They were already far too late in the building process to add one into the CORE's design and had been forced to improvise. The solution had been giving one of the Royal Guard dogs in Snowdin the job of hurling ice blocks into a river that cut through Waterfall and went straight to the CORE. The ice blocks would melt there, keeping the temperature precariously balanced on the edge of overheating. It was a sketchy solution at best, but he couldn't allow them to demolish the CORE just so they could rebuild it and incorporate a cooling system. As long as the ice blocks worked, they were fine.
As the CORE's development steadied and neared its end, and his scientists improved to where he didn't need to constantly monitor them, Gaster was able to slow down enough to listen to the whispers of what was happening in New Home. Asgore and Toriel were ruling, and the town around their castle only continued to grow. However, those weren't the whispers that caught his attention. No, something much more interesting had happened, something Gaster had been waiting for years for and yet simultaneously had given up hope of ever happening.
A human had fallen into the Underground.