After that last, disturbing conversation with Gaster in Waterfall in the room with the disappearing door, for a time, you live in relative peace. Papyrus blossoms under your love and attention, and the brotherly bond between you grows stronger. However, you have frequent bouts of depression, while Papyrus is hurt and confused over how Gaster rejected him, and then abandoned you both.
To Papyrus, it appears that it was his fault, because there was a fight between you and Gaster over what happened with the drawing. You try to tell him that's not the case, and that something's fundamentally wrong with Gaster that's beyond Papyrus's control, but you can tell he doesn't completely believe you.
But your love and care for Papyrus, and his for you, helps to salve both of your inner wounds. While you're both sad sometimes, you manage to be happy much of the time too. You're a small family of two, but still a family.
About a year and a half later, you're sitting at the kitchen table waiting for Papyrus to finish cooking breakfast. You've been teaching him how to cook many kinds of delicacies, and he really enjoys it. He's able to do very well at it when he's calm.
But when he's not...things don't go so well.
Like today. He's standing on a small step stool to reach the stove, where he's making pancakes. He's still shorter than you, but at his rate of growth compared to yours, you guess he'll probably be a head taller than you in a few years.
He seems anxious today though, which is always a bad sign. Maybe it's because of the nightmares he had last night, after which you spent a good hour reading him stories to get him calm enough to go back to sleep.
He works too fast in his efforts to get the pancakes done right, and turns the temperature on the stove way too high. Finally, he presents you with the finished pancakes, and unfortunately, as you suspected, he mostly burnt them. He looks at your face nervously, waiting for your verdict.
"they look great," you tell him. "thanks, pap." The relief in him flows like a waterfall over his expression and frees up the tension in his body, and he smiles. "You're welcome, brother!" he replies proudly.
He sits next to you as you pass him the jam after putting it on your own pancakes, and the two of you sit and eat contentedly. As long as he's happy, you don't care if the pancakes are mostly burnt. Since he made them, to you they're better than if they were made by a king's chef, even if your sense of taste disagrees with you.
He never forgot that moment where Gaster rejected something that he made, and then left you to face the world alone, as if you were both orphans. Now that you're basically his surrogate parent, you think he might be subconsciously trying to make very sure whatever he makes you is as good as possible, thinking that otherwise he might lose you too. That makes him nervous and hyperactive, which makes it hard for him to stay calm when he's cooking.
Of course, you'd never do that to him. You'd never intentionally treat him the way Gaster treated him, and if you did accidentally, you'd apologize right away and never do it again, and you'll definitely never abandon him. But his subconscious doesn't know that.
When the mail comes with the jingling silver bell of the mail monster announcing its arrival, Papyrus pulls on his snow boots and runs out excitedly, bringing in the mail for you. He trails in a lot of snow, but you don't mind. You thank him for his help as he gives you the envelopes.
"YOU ARE MORE THAN WELCOME, BROTHER!" he booms, his voice quite loud for his size.
He grins up at you, and you grin back, feeling especially pleased and happy that he's starting to get his normal voice back. He relaxes, sits back down in his chair again and continues eating his breakfast.
There's a bill or two to pay and a few letters from friends and acquaintances...and one letter with handwriting that part of you hoped you'd never see again. Your peace has been officially interrupted by a letter from Gaster, who is still living at the lab.
For a long moment all you can do is stare at it. You've been hearing dark rumors coming from there of late. Your hands shake as you open the letter.
You're shocked to find it's a curt order for you to bring Papyrus to the lab tomorrow, with nothing in the way of greetings or explanations.
"who the hell does he think he is?" you think furiously.
Papyrus drops his fork with a clatter when he notices your eye sockets have gone black, one of the biggest signs of distress or anger that a skeleton monster can possibly give.
"Sans...?" he questions tentatively.
You've never been more angry in your life, but you put down the letter, take a long, deep breath, and do your best to contain your emotions for his sake.
"it's alrite bro. i just gotta take care of some stuff today. can you hang tough for me while i'm gone? i'll have to leave you with the bunnies for a few hours, but i'll be back by the end of the day to pick you up."
"Oh. Of course, brother!" he says, doing his best to bravely cope with your sudden change in attitude and plans for the day. He can tell you're extremely worried about something and he's trying to lessen your problems by not being a burden to you. You'll have to make it up to him later.
You take Papyrus to a family of bunny monsters who often help you out by looking after Papyrus for free, and ask them to babysit him for the day. The mother of the family agrees and kindly asks after yours and your brother's health, and you reply automatically that you're both doing great. She gives Papyrus a cinnamon roll, and tries to give you one too and engage you in some friendly conversation. You take the cinnamon roll because she insists, but you're in too much of a hurry to stay around and chat. You try to act calm as you make your excuses and leave, so you don't worry Papyrus. He waves at you from where he's already started playing a game of tag with two of the young bunnies, letting you know he'll be fine.
After you leave their home, you immediately shortcut down to the main lab in the middle of the hot lava-filled area that is close to the CORE. As you enter the main floor, the lab assistants quickly get out of your way when they realize your magic is sparking, making it obvious to any monster who sees you that you're getting ready to fight.
You find Alphys, the yellow dinosaur-looking monster who's second in line to be Royal Scientist after Gaster, and who you used to think was a friend before she started avoiding you. You demand to know where Gaster is, showing her the letter as proof that he summoned you.
She shakily points to an odd secret entrance that wasn't there before, and stutters as she explains how to reach his secret underground lab, under the main lab.
"S-sans, p-please don't h-hate m-me! I kn-know it's not r-right, b-but they in-insisted! The k-king and G-gaster, I m-mean."
You glare at her and slightly open your mouth and bare your teeth in anger, a rare expression for you. You know your skeleton monster canines are a little intimidating since they're pretty large and sharp when you're intentionally showing them off like this. But you don't care. That's how angry you are. You can see that something's beyond wrong here just by her guilty expression, even if you don't count the rumors you've been hearing. For Papyrus's sake, you'll fight everyone in the lab if you have to.
She takes a step back from you, looking genuinely afraid of you. You're close to reaching adulthood now, and as the son of a powerful monster, your soul's magic is quite impressive, even without counting your hidden talents. She can sense the true extent of your inner strength when you're prepared for battle like this.
You tell her fiercely, "you could've said no. whatever's going on here, you could've said no. what were they going to do, fire you? is that so hard to live through? being fired? how does that compare with a guilty conscience?"
She hangs her head and rubs her scaly hands together in shame.
"Y-you're r-right," she answers in a small voice, hugging herself with her arms and tail and trying to make herself as small as possible.
You close your mouth, hiding your sharp canines, and try asking her more gently, "come on, alphys. i thought we were friends. give it to me without any bullcrap. i don't have time for it. what's changed you so much that you can't even look me in the eye sockets anymore? what happened to make everyone in the lab look so afraid?"
She keeps her head bowed in shame as if she's lost all sense of self-worth and just wants to disappear.
"It's b-bad. W-when you s-see h-him, p-please, please don't h-hate me!" she says, choking on a sob, and then runs away.
After that strange conversation, you follow her instructions to reach the basement secret lab, entering the elevator, and watching the floors hurtling past you, feeling more and more nervous the closer you get to it.
When you finally reach the lowest level and exit the elevator, the feeling of wrongness is palpable in the air, and fear overwhelms you. Driven by a survival instinct, you move through the hallways with your back against the wall, peeking around corners before you enter the next room, as you move towards what feels like the source of the evil here.
A thrumming fills the air as you enter a large room, and you lose your sense of caution as you gaze in horror at what lies before you.
"This is an abomination," you whisper. A huge vaguely cervine skull shaped out of a reddish steel dominates the room, with large red tubes with diameters larger than your hand leading away from it. In the center, souls are trapped there, behind a transparent spherical barrier of magic contained within the huge jaws of the device. They're the same human souls of six different colors that you gazed at as a child in curiosity. They're leaking red fluid which is apparently being collected underneath them and extracted by the machine. Beneath the machine, there's a huge void rift, like a minature black hole. Underneath that, there's an enormous open pool of red and yellow lava from the CORE's power station, heat rising up from it and visibly making the air wavy. The machine seems to be making use of the rift's energy and the CORE's energy somehow, perhaps as a dual power source.
You can't bear it. Part of you wonders if you should be cautious when it comes to damaging an enormous machine of unknown internal power structure and design, but you decide you can't bear to let this thing to exist for a second longer. Drawing upon all your magical strength, you summon sharp bones twice as large as yourself and use them to impale the machine. It crackles loudly as unstable energy arcs across it. There's a flash of light as a thunderous earsplitting explosion goes off, destroying its insides. Smoke pours from it as alarm bells ring stridently. The rift gradually starts to close as the connection to its power source in the CORE has been destroyed.
The barrier around the human souls ripples and then disappears completely. Suddenly you can hear the human souls screaming, and they're in a lot of pain. Were they screaming this whole time, their telepathic voices trapped behind the barrier before now so no one could hear them? Who could have done this?
You clutch your skull in agony at first as their screams fill your soul, but after several seconds the souls start to feel better and begin thanking you. The glowing soul hearts circle around you as they express their gratitude to you for freeing them. But you also notice that they're trailing the colors of their essence behind them, and growing paler. Are they slowly fading from this world?
"Sans, you fool!" A harsh voice screeches out from behind you, doors banging as two monsters run into the large room. "Don't you know they won't last long in the open with nothing to protect them!"
A strangely disfigured person along with Alphys rushes forward, bringing soul jars in a cart, like those you saw holding the souls before. Using gravity magic, the disfigured person traps them all into the jars again. The souls try to escape from him, but they're too slow. You lift your arm to help the souls escape, but Alphys grabs your arm and shakes her head.
"we c-can't l-let them d-disappear," she says. You glare at her, your eyelids narrowed in suspicion and your expression furious. Did she know about this? For how long?
Once the souls are trapped again in the jars, you can hear their despair, and it haunts your soul. The disfigured person seems to hear them too, because he bends over in agony.
"I can hear them! No no no no, not again, not again! Not their voices! I CAN'T BEAR TO HEAR THEM! Alphys, get them away, GET THEM AWAY! Inside another barrier! QUICKLY!"
Alphys rushes away from the room with the six soul jars on a cart, vanishing quickly behind secure doors. You attempt to follow, running after her, but she pushes you away and locks the doors behind her, and you can't open them. You turn and glare at the strange disfigured person, looking at him for answers.
As you get a good look at him though, you start to realize the awful truth. Part of him are familiar, while other parts of him are horrifically twisted and mishaphen. Half of his skull seems to have melted and reformed into an odd shape, while great holes have been cut into his skeletal hands, as though by a fine cutting tool, intentionally. Most of his body doesn't seem to even completely exist now in physical form. Gaster showed you the void and void monsters in the old days when you worked with him in the lab. That's what seems to be making up most of him now. There's some kind of strange black void energy forming most of his body.
"Dad...?" you ask brokenly. "Why...what have you done? What did you do to those souls? What have you done to yourself?"
You're verging on hysterical, only managing to hold yourself back from losing all control because you know that panicking won't help.
"No one else wanted to volunteer," he says wryly. "We didn't know what Determination would do the first time we tried it. I took samples from myself to see what effects it would have, to make sure it wouldn't destroy me completely."
"You've been experimenting on yourself? You've destroyed yourself! Not only that, you've been hurting the human souls! I thought we agreed hurting them was wrong!" you ask him, feeling bitterly betrayed.
This shouldn't be happening. He should be the one telling you the right thing to do. He's your father, the one you're supposed to be able to look up to.
Despite his disfigured frame, he draws himself up into that imposing stance you know so well from your arguments with him before.
"And I said that I might have to make sacrifices for the greater good of monsterkind," he replies. "Determination may have unfortunate physical effects on monsters, it is true, but it has great potential. We may someday be able to use it to enhance our abilities and break through the barrier. Isn't that worth all of this?"
"Nothing is worth harming an innocent person!" you yell at him. "Any life we had after doing that would forever be tainted! It would have been better for all of us to die, than to have lowered ourselves to this level! I'd rather have died without ever seeing the sun than for you to have done this!"
He blinks, as though he's suddenly lost the mental faculties to respond to complex moral arguments. A moment ago, you were having a coherent conversation with him, but then his eye lights subtly shifted and dulled in a way that reminds you of a saying you read in a book once. The lights are on, but nobody's home.
You shout, "You've harmed six human children! How long have they been down here, suffering like that? You trapped them behind that barrier so you wouldn't have to hear them, right? What kind of person have you become?"
Your voice is naturally softer usually, but you're almost an adult now, and on this occasion, you're so angry your voice is loud and deep as you bellow the words. In fact, "angry" or "furious" aren't words that are adequate anymore. You're not sure what words would be enough to describe how you feel.
Your voice starts to break into a sob with the strength of your emotions, but you won't let it. Sadness won't help. You can mourn the loss of who he used to be later. You need your anger right now.
"Sans, where is Papyrus? I asked you to bring him here," he says in a calm, authoritative tone, as though he didn't hear anything you just said.
"You're not getting anywhere near him!" you snarl.
He looks at you with a condescending air, but now that his face is so disfigured he can't pull it off anymore like he used to. He just looks like a sad excuse for a monster now, like something out of a nightmare that shouldn't exist.
"Are you saying you wish to take his place? Sans, we both know your health is too low to be a useful experimental subject."
Your expression settles into the coldest death grin anyone has ever seen on you. After a few seconds where Gaster doesn't react to it, you know for sure the father you used to know isn't there anymore. The old Gaster would've quickly picked up on your mood and realized what a mistake he just made. But now, he's too far gone.
"Is that what you were going to do to him?" you ask threateningly. "Not. Happening." you say, biting off each word with a snap of your jaws, baring your canines. Both your eye lights are a raging fire of bright blue and yellow magic, and the magic lines that run throughout your bones are overflowing with energy.
"Not now, not ever. I won't let you anywhere near him!"
Gaster doesn't seem perturbed. "You are saying you do wish to take his place? Very well, but I must take some measurements first to see how well suited your soul is. Void energy helps keep determination under control, but the combination of the two can still have some unfortunate effects. I wouldn't want you to die before the experiments are over."
He attempts to grab you with gravity magic, but you see it coming from past duels practicing with him in the old days, and you dodge away.
"Sorry, dad, but you should know I'm not that dumb," you tell him. "I'll kill you before I let you near either me or Papyrus. If something happened to me, who would protect him from you?"
"Stop resisting," he says in a level tone. "I don't want to damage you more than I have to. You'll contaminate the results of the experiment if you resist, and I'll have to do this all over again."
"You're not our father anymore," you yell at him. "Go to hell!"
"Hell is like a deep darkness, where strange beings roam. Dark, darker...yet darker..." he replies in a sing-song, insane-sounding voice.
His tone changes, and he almost sounds reasonable. "Don't you want me to save the underground, Sans? What are a few lives compared to that? Meaningless."
"You tortured those human children! I'll never let you do to him what you did to them!"
His eye lights flare with intense emotion and purple fire at your words.
"They're just...they're just humans," he says. "Enemies. Their kind trapped us here, so we have the right to do whatever we want to them if it helps us escape..." He sounds completely and utterly unconvinced by his own words.
He tries to justify himself to you weakly. "If I...if I let him have them, they'll both die, the king and the the souls...can't resolve it! Can't resolve the paradox!"
Then his expression hardens, and he speaks coldly as if answering himself.
"Then just shut up, and let me take over. You're so pathetic. So incompetent. All you do is mess everything up. If you don't want to feel hurt, then go back to sleep, and don't interfere with me anymore."
Whirling void rifts appear on each side of him as he brings out his Gaster Blasters, firing twin shots at you. But if there's one thing he taught you how to do when he used to be sane, it's how to dodge.
He said back then, when he was training you to fight, "Your one and only hope in a battle with a human is to dodge attacks perfectly each and every time. You can't let a human with their malice get a solid hit on you even once, or you could die. Their malice is that lethal to monsters." Who knew you'd be using that dodging skill you worked so hard for, against him?
You keep dodging his attacks, which are a combination of gravity attacks, Gaster Blasters, and bones, while you look for a crack in his defenses. But he's too good. You're strong, but he's a powerful monster with decades of experience behind him.
He sends a sharp bone attack your way, and you almost manage to hold it back with gravity magic, but its momentum is slightly more than you expected. It partially pierces your right eye socket, and you scream in agony as it pierces the center of your blue and yellow eye light.
If it had gone any farther in, it would've pierced the core of magic in your skull and shattered your skull, and no skeleton can survive that. As it is, you've probably permanently lost most of a skeleton's normal magical abilities in your right eye light. You know that a skeleton's soul is heavily connected to their skull, and particularly to their eye lights, so it's a terrible loss.
Reflexively, you dodge backwards and away from the sharp bone piercing your eye light, as another bone hits the spot where you were a moment before. A split second afterward, Gaster Blaster attacks hit there as well. As you expected, Gaster tried to use your moment of weakness to attack you.
Gaster's attacks intensify, and you try to recover and keep dodging, despite the intense pain in your eye socket and the loss of half your battle vision. It's incredibly difficult to keep up, and you're tiring. Your stamina is not great, even at the best of times, and you've been wounded.
You pant heavily with the effort of dodging his attacks. You can't last forever against him. You have to think of something.
Suddenly you get an idea. Maybe you can use his guilt against him. It's obvious he's not mentally stable. You just have to break his concentration on the battle.
You speak with your teeth closed, and you project your voice against a wall so that it reflects and seems to come from a different angle than from you, so it's less obvious to him that it's you talking.
"Why are you keeping us trapped in here?" you say, trying to mimic the voices of the human souls. "Why are you doing this? It's wrong to keep us here. Please! We just want to go home!"
Gaster crumbles mentally before your eyes. "NO! LEAVE ME ALONE! THE VOICES! STOP THE WHISPERING VOICES!" he screams, rocking from side to side.
At that moment, you feel a new skill rise up in your soul. It wants to fight for you. You're not sure what it is, but you manifest it and throw it at him, hoping it'll be something good. It turns into a sharp purple bone and embeds itself deep in Gaster's soul. He screams in pain, and his HP drops by a very large amount, so that he has very little of it left. You sense the skill searching for the memories of guilt in his soul, and reminding him of each of them. You can't see the details of the memories yourself, but you sense his reaction to them. It feels like the skill is punishing him for every past action in his huge debt of karma.
That worked better than you thought it would. It gives you time for your next moves. Carefully, you form normal white bones and use them to confine him in a bone cage, further restricting his magic. You form blue bones going through the cage at different angles so he can't move without taking damage. He tries to strike out at the bone cage and destroy it with his own bone attacks, but it's not enough after the purple karma bone and the blue bones have weakened him.
All of this has heavily depleted your own magic. You have to act quickly. While he's distracted, trapped, and weakened, you grab him with gravity magic, and throw him towards the void rift under the huge metal jaws.
It's the only way you can think of to stop him without outright killing him. There's a chance he might survive in the void. You hesitate though, and your hand stops moving, and your gravity magic halts his soul's momentum toward the rift.
Your soul hurts. You don't want to do this. You're not sure what will happen to Gaster in the void, but he'll likely suffer a horrible fate in there, if he manages to survive at all, which is questionable. You consider pulling him back from the rift, but you can't think of any other way to protect Papyrus.
If only you weren't alone in this decision. If only Asgore or someone else powerful had helped you capture Gaster safely and imprison him somewhere he couldn't harm anyone.
Asgore, Alphys, the other lab assistants...none of them stood up and prevented things from getting so out of control. You can't trust them to help you contain Gaster's insanity before he hurts Papyrus.
As you hesitate, you notice that he's desperately trying to communicate something to you with his eye lights, and it doesn't match the rest of his expression and body language. He's pleading with you, and it doesn't feel like he's asking you to stop and pull him back, but to keep going.
When you see that, you remember that your father gave you his consent to do what you have to if it ever came to this.
"At least I'm keeping the two of you away from this. If I ever stop doing that, you'll know the real me is lost."
That's what he said. He never wanted to hurt either of you. It's the worst thing you've ever had to do, but at least if you can manage to confine him in void space, there's a chance he'll live through this, and Papyrus will be safe from him.
You have to take this chance while you have it, before he breaks the bone cage apart. You push his soul with blue gravity magic into the void rift. He resists you with all his strength from inside the bone cage, and his magic tears at the edges of the rift, making it more and more unstable. With a final massive push, you manage to force him inside, releasing him from the bone cage at the same time. He screams one final time, and it's a shrill, alien, panicked sound, not like a skeleton monster at all. The rift closes and shrinks rapidly into a twisting, spinning ball of energy, like a miniature star readying itself for a supernova.
Panting heavily as you watch the rift, you have a split second thought to shortcut away from the impending explosion, but there's not enough time or magic left. Your right eye socket hurts like hell, and your soul has used up too much of its magic in the fight. You brace yourself and shield your eye sockets with your arm as the rift explodes with a boom, sending out a fiery heat wave that sears your bones. A final, powerful wave of void energy follows afterward, before the rift shrinks to a pinprick and disappears for good. When the void energy hits you, your mind feels strange, like a null space filled with static. You sway unsteadily for a moment, before falling to the floor with a harsh bone clattering sound.
As you lay there on the hard cement floor, it feels like black ghostly shapes are tearing at your mind, trying to rip memories away. You grit your teeth and refuse to let them have their way, struggling to hold onto consciousness. You know you've taken so much damage that you could easily die right now, but you will your soul to keep from breaking apart. You refuse to die or to let the void steal any pieces of your soul away from you.
"papyrus..." you whisper softly. Keeping him in your mind makes your will to live stronger, and you hold onto your thoughts of him as much as you can. You wonder how he's doing with the bunny monsters, and then you remember the cinnamon roll in your pocket. You gratefully take it out and chow down on it. The healing magic in the cinnamon roll spreads out through your bones, focusing mostly in your right eye socket where your worst injury is. As the healing magic works on it, that area aches intensely but in a good way. Your right eye socket and eye light and some of the burns throughout your body from the void rift's explosion are partially healed by the food's magic, but it still hurts a lot. For what feels like a long time, you curl up and try to deal with the pain and stay alive.
You hear footsteps cautiously approach you.