Chapter 1: 🫀 The Heart
Chapter Text
Doctor W. D. Gaster could feel the air on his skin, warm and heavy.
He stood outside the CORE, leaning on one of the metal railings that hung over the side of what was the largest creation he had ever made in his life, looking at the horizon over the lava lake, at the direction where Waterfall and Snowdin were.
Not even a month after he turned it on, and some monsters were already calling him the best scientist to ever exist. The best monster to ever exist. He even heard someone calling him “the greatest creature to ever exist.”
He took those compliments with a smile and treasured them into his heart. That’s where they belonged: Pure appreciation, absolute happiness. Expressions of hope, tears of joy. Those emotions weren’t something people gave easily, and Gaster knew it.
He forced a smile onto his face. That was it. His biggest accomplishment, done. He gave an entire race the possibility to have a future, even in the circumstances they were. Light, electricity, warmth, movement, automatization, construction… Everything was possible again.
He forced the smile even more. That was something to be proud of. And he would be proud of it. He would not let the other monsters down because of his own insecurities.
This is the most dangerous thing that has ever been built in-
Gaster scattered that thought away. His smile trembled, but he could restore it just in time. What a foolish thing to think about. A scientist, afraid of his own creation? The laughter that that would provoke will for sure be heard over all the Underground.
And after that, the sorrow…
No. Gaster wasn’t done yet, and he had to endure it. His job didn’t end there, so he started to walk inside the building. Too much time alone was dangerous. His brain wasn’t as reliable as it used to be.
The next step of his plan was to make the maintenance of the CORE self-sufficient, thanks to all that magic energy that was flowing through the pipes and establish the emergency protocols in case something went wrong. The rest of the monsters didn’t know this, but electricity wasn’t the only thing the CORE was capable of.
And before this was released to the world, Gaster had to make sure that his creation would not destroy it.
His co-workers were inside, making sure everything was calibrated correctly. Walking fast through the hot hallways, he reached the control room, where all the devices and consoles that controlled the CORE were located.
We will need a way to refrigerate the building, Gaster thought as he opened the door.
A cold snap hit his face, followed by the looks of his three co-workers.
“Close the door!” yelled one of them from a chair, sitting right in front of a device controller with some screens and buttons.
Gaster followed the order, so he closed the door as fast as he could without hitting it with too much force.
“Phew!” exhaled the same co-worker. “Sorry, Gast, but the temperature out there is insufferable. And we thought about putting the air conditioner to its maximum to counter it.”
“I’m surprised you haven’t become ice blocks with this temperature,” Gaster said while he ran to turn the temperature up, his teeth starting to chatter. “Also, Fili, do you know how much energy an air conditioner uses?”
Fili shrugged. He was so little that he almost had no shoulders in his orange body to shrug with, which made it comical to see.
“We literally have an infinite source of energy right below us, so we thought that that wouldn’t matter.”
“No! YOU thought that!” exclaimed another co-worker that was at a folding picnic table eating some muffins. He was taller, with a more humanoid form and green skin. “This isn’t an infinite source of energy, but an EXORBITANT source of energy. They are NOT the same!”
Fili rolled his eyes.
“Scadogu. Please. At the first week after this thing was turned on, we were creating so much energy that we were considering the possibility to build some pipes to expel it outside the building, because we didn’t know what to do with it.”
“But we KNOW, now!” Scadogu replied. “This is the most beautiful, powerful and dangerous thing ever created in the history of this world, and EVEN then, we have to talk about it properly. It is our job as the co-founders AND maintenance team.”
Not even here I can take a breath, Gaster thought, sighing.
The third and last co-worker finally spoke. Was a little hidden, under the device desk, making sure everything was connected correctly.
“Look at how exhausted Gaster is!” he exclaimed while coming out from there. This co-worker was a bird with cyan blue and green body, but with feet so minuscule that looked like he was sprouting out of the ground. “He is more heated up from you both than from the heat of the CORE. The award that Asgore gave him should also include a medal to the most patient monster in the whole Underground!”
“Thank you, Tafa” Gaster said, trying to remain calm, “but I don’t want to escalate the situation. Actually, the reason I’m here is because I’m not feeling too well, and I think I’ll go back to the laboratory. So, you’re responsible for the maintenance of the CORE while I’m absent.”
“You’re not feeling well?” Fili asked, rotating the chair to look at Gaster directly. “Well, I suppose that you’ve been exposed to too much heat out there. Maybe staying here with us will be enough.”
“No, no… I mean, maybe that would do, but I have some experiments to do, and I don’t want to delay them any longer.”
“Doc, are you HEARING yourself?” Scadogu replied. “There’s been only a month since we turned on this thing, and you’ve been NON-STOP, going and doing everything to ensure that nothing fails or breaks. Maybe it’s time to REST, don’t you think?”
“Yeah!” Tafa intervened. “You are not above anyone else, Gaster. The only times I’ve seen you in this room, none of them were spent eating or drinking something, or reading a book that wasn’t related to some scientific matter. You should really look for yourself more often, my friend.”
Gaster was hearing all those praises, concerns and advices, and felt his heart melting with warming love. Those were very similar compliments that the other monsters were saying, with the intrinsic recognition that he was someone to care about, someone to listen to, someone to take advice from.
But there were also the worries of his own team. Gaster should not look weak or sad. Why? He made it into history. His name would be heard for ages, and his work had the capacity to revolutionise the whole world, not just the Underground. His co-workers knew something about it, something related to expand the capacities of the CORE, but that burden was only his to endure.
He forced again the smile. He was proud of all of this, right? Well, he’ll look like it.
“I’m terribly sorry to worry all of you like that” Gaster said, laughing. “But I’m better than I’ve implied. Seriously! Nothing to worry about. It must be the age, catching up to me.”
The three co-workers exchanged some concerned looks.
“You sure, Gaster?” Fili asked. “We can go do something else while you rest here. Really.”
“No, no, seriously!” Gaster ensured, enhancing his smile even more. He was very happy and motivated about all of that, and they had to see it. “At the lab, I can rest if I need to. I’ll be much better there, and tomorrow I’ll come here better than ever!”
Fortunately, Tifi believed him and backed him up.
“Guys, I think Gaster is the one that knows better what he’s capable of” he said. “Give him the benefit of the doubt. If not, we’ll spank him until his face starts going full rainbow mode! Right, Gaster?”
Gaster laughed again.
“Yeah, absolutely! But I repeat: you have nothing to worry about. Now, I will go to the lab right now before you threaten me again” said with a chuckle. “Take care of everything while I’m gone, and I’ll see you tomorrow!”
“Sure THING, boss!”, Scadogu ensured. “And don’t beat yourself TOO hard. You’ve done MORE than enough.”
Gaster thanked him with the smile and a wave, and then left before anyone could say another thing about him.
While he went downstairs and made his way to the lab, his heart and brain felt warm and heavy inside him. Those worries that his own co-workers, and all the people in the Underground had about him… he deserved them because all he did. That was the main reason.
But they should worry about being happy and enjoying their new and bright world. He didn’t want to overshadow all that happiness with worried looks. It was his duty. And he knew that. Everyone should go back to being charming and celebrating their new future with shining hope for their dreams.
I will make sure to keep being worthy of them. Not a monster in the Underground will need to worry about anything anymore. Not even me.
He would find a way to make the CORE safer and use all his capacity to good use. All the danger that it could have was only in his brain, overthinking. Maybe he was tired, after all. Yeah, that must be that. But because of it, he knew for sure that all peril was undoubtedly unfounded.
Things were better than he could see at that moment. He was smiling, right? Yeah, he was smiling.
Because everything would be fine.
Notes:
Hi! You’ve read the whole thing? WOW, THANK YOU SO SO MUCH, SERIOUSLY. FROM THE BOTTOM OF MY HEART, THANK YOU!!!!!!!
I was so nervous to publish this, because again, this is my first story in English, and my main language is Spanish. I didn’t want to make something too complex or long for my first try, and I wanted to write it about a fandom I know and love to make it easier. And this fandom is UNDERTALE!
I wanted to write something worth this videogame, which hit me like a truck and made me cry so hard, and I didn’t know if I could do it in English, but something inside me was urging to try it and bring it something to life, anything.
And so, there it is! I really hope you liked it, and I’ll see you in the next two chapters!
Chapter 2: 🧠 The Brain
Chapter Text
Four hours passed since Gaster left the CORE. It was only 10 pm. But Gaster already knew he would not sleep that night. How could he, when the fate of the whole Underground… no, the whole WORLD was in his hands?
He just needed to be careful to not let it slip through the holes they had.
Deep under his lab, where even the light of the fluorescent tubes that hanged from the ceiling seemed to struggle to illuminate enough to make any view comfortable, Gaster was trying to not lose his mind while that horrible future he visualised was becoming more crystal clear at every second.
The end of the humans. The end of the monsters. The end of the world.
He tried not to panic and focus on his work. All night was spend configuring a machine that was able to replicate (at a smaller scale) the unique properties of the CORE. That artifact was created years ago by himself, when he started messing around with the idea that, eventually, would become the major achievement of his life. He just needed to put it back to work so he could test some things, but…
“you sure this thing works, gast?” asked a voice behind him. “there’s more dust than cables down there. you’re lucky i don’t have any lungs to fill with all this dirt so i can’t complain about this breathtaking task you’ve given to me.”
Gaster frowned.
“Sans,” said the doctor without turning around, “just make sure everything is at it’s place. We need to run this experiment at the best of the conditions. Nothing can be left to interpretation.”
“yeah yeah, i get it. it’s just… well, i thought you already forgot about this device. i was surprised you didn’t even dismantle it. you just left it here, gathering dust, as…”
“Sans.” Gaster snapped. “Get. Back. To work.”
Silence.
“… ’kay, buddy.” Sans responded, and continued cleaning the insides of the machine. It had the appearance of a giant animal skull. Sans always joked about it as a result from “the love to his job, that went to the bone”. Gaster never replied to those comments.
The doctor was calibrating the monitors, the sensors and the different buttons, levers and lights that made up the control table that steered the grim machine. For safety reasons, it was placed in another room, one built behind the room with the animal-skull-shaped-machine.
The walls protected Gaster from the possible side effects of the machine, yeah. But…
Gaster heard Sans’s steps approaching him from behind.
“it’s done, doc.” said with a tired voice.
That was the first time Gaster turned around to see his friend. All his body was drenched in sweat, his lab coat soiled and wet, and was breathing with his mouth open, as if he was trying to catch his breath.
Gaster blinked. Sans looked very exhausted. Was it because the task he gave him? Was it really that exhausting? Sans was lazy, yeah, and doing exercise wasn’t something he did at all. Moreover, he didn’t have any muscle to exercise in the first place. His heart shrinked a little bit, anguished.
No, he thought. You are very exhausted, too. It’s not like you’re putting all the weight of your responsibilities onto Sans. You’re also giving it your all to bring the brightest future to the Underground. This is a shared duty, one that, at the end, will make that no one worries about anything anymore.
“At my order, pull the lever two levels up.” Gater said while leaving the room. “After that, put the lever one level up every time I shout at you.”
The thing was: the CORE had the capacity to extract the heat from Earth and turn it into electricity. That was the explanation that was given to the public and Asgore.
But Gaster knew more, and his co-workers, Sans and Alphys had started to suspect that. The CORE was not limited to extract heat. The CORE was not limited to convert it into electricity.
He made sure Sans did hid work well by crawling inside the machine and looking at it by himself. Yeah, Sans did a pretty decent job.
Then, got out of there, went in front of the metallic mouth of his creation and put a magic marker right there, on the floor.
“Up!” he shouted, giving Sans the order while taking a few steps back away from where the magic marker was.
The eyes of the skull-shaped machine turned on with bright white light, and the sound of metal pieces coming alive almost left Gaster deaf. He almost didn’t care.
Then, the machine made another sound. A sound that always gave Gaster the most unpleasant chills he had ever felt. A sound that he could only describe as “the void, taking form”. That was the best way he could manage to explain how that sound made him feel.
Shortly after that, the magic marker started to loose it’s color. There was no light, no sparkles, no visual indicator that could teel what was happening. But Gaster knew it.
He knew it too well.
The marker turned gradually grey within a minute, and then, completely white.
Gaster focused on the marker, and without taking his eyes of it, shouted at Sans again:
“Up!”
After a few seconds, the sound became louder. Gaster felt goosebumps rise on his skin.
And then, the marker started melting.
Gaster’s heart was pounding violently in his chest, but he forced himself to ignore it. The doubts and the questions he had in his mind craved for answers, and he was going to find them.
Through the drowning, through the pain, through the fear. Through the barrier itself, if needed. He would make their hopes and dreams become true. It was his bidding, as the Royal Scientist, and “the greatest creature to ever exist.”
They thought that way about him, didn’t they? Well, he would not be a disappointment or prove anyone wrong.
The magic marker became a pale soup, completely unrecognisable from what it was minutes ago.
“Stop!” Gaster shouted, and all the horrible sounds started fading away, slowly but steady. While the machine was entering his dormant state, he entered the room where Sans was and went directly at the graphs that one big printer started to spit out.
After looking carefully but hurrying at the long paper with various lines of different colors, went to the wall that separated that room from the machine, and turned on some big television that was hanging from the wall.
And a sphere of shadows appeared in front of a bright white background.
Sans didn't say anything, he recognised the phenomenon. But his eyes went wide open when he discovered how big and strange it became after all those years. The sphere was spouting shadows, thin as a piece of paper, that traced an arc and slowly fell back into the darkness. It was not chaotic, it was calm and paused, but nothing remained at the same place for more that a few seconds.
Gaster quickly took out a voice-recording device out of his pocket and started talking to it.
ENTRY NUMBER ONE
THE VOID CONSUMED IT ALL
THE SHADOWS KEEP GETTING SHARPER
THIN AS PAPER, LIGHT AS AIR
MAGIC TRACES: NON EXISTENT
LIGHT TRACES: NON EXISTENT
MATTER TRACES: NON EXISTENT
STABILISED AND CALM
READY TO CHANGE
…
READY TO BECOME
…
Gaster cut the recording and groaned with frustration.
“I need to complete it later,” muttered as he turned and went quickly to the control device. He didn't seemed to acknowledge Sans as he walked across the room, and the skeleton took a step back, in silence.
Expecting something, he only received silence from the doctor. The only sound came from the quick and precise typing as Gaster wrote something very long on the computer.
Sans cleared his throat. The doctor didn't even blink. Sans worried a little, and started to talk.
“doc, i-”
“Sans, clean the soap that was the magic marker,” Gaster spouted without looking at him.
Sans remained in place, waiting for a reason or more indications, but he didn't receive any. Almost immediately after that, Gaster turned around quickly and collided with Sans, but managed to not fall on the floor.
“What the…?!” Gaster looked down, and finally took notice of Sans’s presence. His face went red. “Sans! What did I tell you? Go clean that mess now!”
Sans was shocked for a second.
“hey, doc, you shou-”
“NOW!”
Sans took a step back instinctively, and without a word, turned around and exited the room. Gaster, still fuming, went back to the graphics and took some notes on a notepad that he always kept in his pocket.
Stupid lazy bones, he thought. As if I didn't have enough already. I'm doing all the work, and all he does is complain and get on my nerves. The world could end tomorrow, and he wouldn't even blink.
He felt a twinge of shame inside his heart, piercing across his anger. He never talked to Sans like that.
His hand stopped writing.
Gaster was doing all of that for people like Sans. He was one of the monsters he promised to take care of. That was the whole reason for all of that. He had to remember it.
Did he cross a line with that outburst? Should he relax a little, too? Actually, his co-workers at the CORE were talking about that. The whole thing was really messing with him right now. What if… what if he started harming the people he was so focused to protect?
He thought about them, and then looked at his left. The TV was still on, with the shadow orb, or the Void, as he called it, moving and changing on the screen, slow but steadily. Gaster knew that wasn't actually a real void that could absorb everything. No, that was different. And was also dangerous, but for a different reason.
The experiment that he was doing, the machine, the Void, the CORE… A shiver crawled down his spine. The world could be in imminent danger, and he was the only one who could do anything. Even more, he was the one who put the whole world in imminent danger.
His anger grew again, suffocating that twinge of shame. He was wasting time. The way to protect everything and everyone from the CORE and from himself was doing something, and he had to do it now. Sans would forgive him. A price needed to be paid for the safest and brightest future possible, and he already invested too much to hold anything back.
He really cared about everyone. If that meant sacrificing himself… well, the sunk cost fallacy agreed with him. Too late to think about that, now.
He already sacrificed too much to let that thought affect him.
Notes:
Thanks to Amina for doing the beta reading of this chapter ✨