Gaster has a good head start on the others. He moves through the core quickly, still carrying Flowey who's completely immobilized within the magic surrounding him. He ducks into a room towards the center of the core, taps a few numbers into a computer terminal and stands back as a secret door is opened. Flowey stares at it as the pathway is opened towards what looks like it could be the very heart of the machine. Rows and rows of light filled glass tubes line the walls, and at the very back and center of the room is a small glass chamber hooked up to all of them. Gaster puts Flowey into it and closes it before he can escape.
Flowey, finally able to move again, unleashes a furious attack of bullets on the glass. They bounce off the glass harmlessly, making him have to duck and dodge away from the ricocheting bullets. Gaster looks at him and shakes his head. "The glass is meant to contain magic. There's no point in what you're doing."
Flowey looked confused. "W- what?"
Gaster sighs and types on the computer, and it speaks aloud for him. "I said, what you're doing is pointless. The glass is a special crystal compound that reflects magic." He minimizes the text to speech program and starts working on something else.
"What… are you doing?" Flowey asks.
Gaster looks up from the computer briefly, then looks back down continuing his work.
"I asked you a question!"
"I heard you," Gaster replies with the computer. "Don't worry about it. It will be over soon enough."
"A… am I going to die?" Flowey asks.
Gaster considers it, "Hm, can a creature without a soul truly die? Are they alive to begin with?"
"Yes!" Flowey shouts, standing up taller as though to exaggerate his own existence. "So… you don't have to do this."
"Unfortunately… I do, you'll have to understand, it's for the good of the world," Gaster says as he gestures briefly over to a lever on the wall before continuing to type. "Once I pull this, all the energy stored in the core will be transferred to you… all of that magical energy should be more than equivalent to the energy of seven human souls, more than enough to shatter the barrier."
"But… what does the barrier have to do with anything?"
"It's where the cracks end… for whatever reason the breaking of the barrier is a convergence point. When it goes down, I will have just a moment to seal them and put my timeline back where it should be."
Flowey looks down, eyes fixed to a point far beyond the glass confines of the machine he's sealed into. "But… what happens to this timeline?"
Gaster shrugs. "It was never meant to be, and so it won't be. Nothing to worry about, really, I'm sure the version of you from the original timeline is doing much better than being trapped in a flower."
"But that isn't me, that's a different Asriel," Flowey protests. Gaster doesn't seem to care, looking at the lever. "I… I won't help you, you know. I won't break the barrier if that's what you're planning to do!"
"Oh you poor little Prince. Think to yourself, from the moment your little friend gained the power of determination, the power of saving, of resetting… Once all that occurred… since when were you the one in control?" Flowey stares at him, and in truth, he probably has no idea. "You will do as I direct you to. The energy given to you will be too overwhelming by far for you to make any conscious decisions of your own."
Flowey's determination to stay strong begins to break down. "Please don't do this, please don't… please…" Gaster shakes his head and wordlessly walks over and pulls the lever. "No!" In a last attempt to free himself, Flowey sends a volley of bullets against the glass. It changes nothing.
The power flows into him, filling the chamber with blinding light.
"it's this way," Sans says, leading them forward. He ends up skidding to a halt when he's faced with a dead end. Papyrus, who had been keeping close to him nearly crashes into him, and Frisk nearly crashes into Papyrus, before Undyne yanks their shirt back and stops the chain of collisions before it can start. Sans stares at the wall, puzzled. "at least i thought it was…"
"Sans, if you'd t- tell me where you're trying to go, I might be able to help get us there," Alphys points out.
Sans shakes his head. "no, he never showed this room to you, not even in the other timeline…" He looks around. "but it should be near here…"
"What should?" Undyne asks.
"energy storage," Sans says, looking around. He searches a few more corridors and eventually come across what looks like an empty room and grins. "here we are," he says, walking to a computer and typing in it. The back wall opens up, and the room, the glass tubes, and the chamber in the back are all empty. "oh no…" he says quietly.
"SANS, THERE IS NOTHING HERE!" Papyrus says. Frisk meanwhile finds the crudely painted flower pot and picks it up in silence.
"yeah, i know," Sans replies. "that must mean he's already been here… we need to hurry to the barrier."
Getting to "New Home" is no hard task for Gaster. He points through an empty hallway in which large glass windows letting a soft, golden light through. But that changes quickly when Asriel steps into the cooridor. Around him, the final product of years of storing vast quantities of magic are making his small form shine like a beacon, drowning out any other source of light in the room. "Right this way," Gaster says to him, though there is no indication that Asriel can understand. He simply continues to walk down the hallway.
Gaster moves ahead, into the hall leading to the throne room and leads him in in silence. Not too far ahead of them, a large, horned monster is turned away, watering golden flowers. Asriel takes another step forward and in front of them, the monster's ears perk as he hears the sound. His cloak makes a slight sweeping noise as he turns to greet them.
"Hello," he says in a deep, rumbling voice. "Sorry, I didn't realize I'd have vis-" As he sees who's in the room, his mouth falls open and the watering can he was holding drops to the ground. The sound echoes through the silent room. "Asriel?" the king whispers, blinking his eyes as though he's certain they're deceiving him.
His son doesn't answer, staring ahead blankly with empty, glowing eyes. Gaster, meanwhile, steps forward, gesturing for Asgore to move aside. King Dreemurr stands his ground, and Gaster rolls his eyes, directing Asriel forward. He obeys without thought.
"What have you done with him?" Asgore asks. Gaster doesn't answer, eyes glowing as a warning. Asgore draws his trident, unintimidated. "We don't have to do this, but you shouldn't move forward any further… whoever you are." Gaster takes a step forward and Asgore sighs a little sadly. "I see… well then…"
In a flash he swings his trident at Gaster. Gaster ducks under it once, and before it's swung again, he lifts a hand and stops it dead in its tracks. Asgore pulls the trident back, and rings of fire magic launch at him. Gaster raises a hand, pulling bricks away from a wall and shielding himself. He then retaliates, throwing the bricks at Asgore.
Asgore puts up a valiant effort to stop Gaster, but it's little use. Eventually, there's nothing Asgore can do but step between Gaster and Asriel, shielding him with his trident.
Gaster sighs. "You really think that's going to help? You won't stop me now," he says moving forward.
Suddenly a burst of flame hits Gaster, knocking him away from Asriel slightly.
"What a miserable creature… what have you done here?" A soft but protective voice asks, sounding unusually clear in the silence that surrounds them. King Asgore looks towards the entrance of the throne room in shock as Toriel steps forward and sends a burst of fire magic at Gaster. He has to dodge quickly just to avoid being charred.
Asgore stares for a moment. "Tori!?"
Toriel's determined expression falters slightly at his voice, lapsing into annoyance, but she quickly recovers it, throwing another volley of fire against Gaster, trying to get to Asgore and Asriel. "Asriel…" she calls out to him. The prince looks up at her without recognition.
Gaster looks between the three them. "A family reunion, is it?" he asks. He throws Asgore towards the opposite wall and puts his arm in front of Asriel.
"Let him go!" Toriel says.
Gaster's left eye glows brightly and blue magic surrounds Asriel's hand, lifting it up. He swings it and a bright beam of light knocks Toriel away. "Tori!" Asgore calls out to her, but he's soon subjected to the same attack. They both tumble into the wall of the garden.
Gaster allows Asriel to lower his hand, but keeps his magic up, moving him forward. "Almost there… almost there…" he murmurs to himself.
Toriel has just enough strength left to push herself up and see Gaster disappear behind the throne, leading her son away.
The group reaches the throne room not too much later after Gaster leaves it. Sans rushes in to look for him, but he's already moved on to the next room. Undyne spots Toriel and Asgore and turns to Sans. "Hey you and Papyrus go ahead, we'll catch up." Sans nods, and the two skeleton brothers leave.
Frisk runs to Toriel, helping her up. "Oh… thank you, my child," she says, sounding a little sore. Her eyes are fixed on a heart shaped locket that Frisk must have discreetly picked up on their way from The Core. She turns to the king. "Asgore, get up, we need to go. Now." Asgore stands quickly, wincing.
"Oh good, we have more backup," Undyne says to Asgore, before taking a better look at Toriel. "Wait… that your ex? What's she doing here?"
"I have no idea… Tori, how did you know to come?" Asgore asks.
"Well, I found something odd in the ruins, a gray door… and… well I can explain later, we need to catch up."
"You're right," Asgore says. "The barrier is this way everyone." He leads them all forward.
"It's all come down to this," Gaster says quietly, looking out at the brightness of the barrier. Though Sans and Papyrus had both tried to slow him further, he'd tossed them aside effortlessly. "One last obstacle… once this goes down, the effect will ripple back to the original timeline… And I'll have finally accomplished what I truly wanted to do with the core. …Isn't that exciting, Asriel?"
Asriel doesn't respond. The white hot energy that now made up his entire being was swirling around him, a supernova of light and magic. He looks towards the barrier with eyes fixed straight ahead.
"Well… You know what you have to do," Gaster says, gesturing forward. Without true comprehension, Asriel nods slowly and starts walking forward, each step leaving the ground glowing in his wake. He walks nearly halfway to the barrier when, far behind him, Frisk enters the room.
*Asriel! Asriel?
He turns around, looking back at them. They're clutching their tiny toy knife, though after all this time, I can't picture them using it. Around their neck, Asriel seems to be transfixed on the heart-shaped locket. "Ch… ar… a?" he asks slowly. His voice echoes around the room.
Frisk shakes their head, but goes to him, holding a hand out.
Asriel stares at them, when suddenly a dim recollection is evident in his brilliantly shining eyes. "Frisk…"
Asriel, you have to come back with me. *Please come back!
He stares at them without understanding what they're saying to him.
"Asriel, the barrier," Gaster says, turning him towards it.
"Asriel!" Toriel calls out. The king stands with her, trident gripped in his hand.
Asriel looks around himself, confused, and covers his ears, crouching into a ball. Gaster sighs in annoyance and pushes him towards the barrier. Undyne and Asgore each rush to stop him but he throws them aside as though they are nothing. Asgore looks up as Asriel touches the barrier.
As soon as he makes contact, he's lifted into the air, and the barrier brightens as the energy diffuses around him. Gaster's eyes flash red and blue and a crashing sound is heard from high above. Frisk looks up as a crack forms in the sky. Everything is still for a moment before the very earth around them begins to shake, and small pieces of what looks like light solidified rain down from crack in the sky. They crash to the earth with a shattering sound.
It's then that I realize that they were much too late to stop what was to come.
The entire world is ending. Larger and larger pieces of light are raining down as the ground continues to shake. Frisk holds their hand over their heart, looking at the cracks above and around them. Undyne turns to Alphys; they hold each other tightly. Toriel looks up, not at the sky, but at her son, suspended in the air as everything topples around him. Asgore can't bring himself to look at anything at all. This was never what he wanted, this was never what he had expected. Sans tightens a fist, but he's looking as defeated as he has always felt.
"SANS… THIS… THIS IS IT?" Papyrus asks. For someone normally as cheerful as him, his shaking voice was disheartening.
Sans can't bring himself to answer that. He marches towards Gaster. "why?" he demands with surprising forcefulness. "why would you do this? why does it matter if any of this is right? why!?"
Gaster looks over everyone. "I… had to do it to save our timeline, Sans. Surely you of all people would understand that."
"you aren't saving a timeline, w.d., you're destroying one! look at this, look around you! how can any of this be right?" he asks gesturing at the other monsters, but then he looks away, rubbing angrily at his eyes. "i guess it doesn't matter now, does it? it's all over, everything is over. i'll never see 'em again… once was bad enough…"
Gaster tilts his head slightly. "You… truly have come to care for all this?"
"of course i have… this is my home. or it… was my home," he sighs.
"It wasn't. I can take you back to our old-" he starts, but Sans cuts him off with a sharp glare. It manages to look horribly intimidating, even without the magical glow to back it up.
"I won't go," he says firmly. He looks to Toriel, so overcome by the sight of seeing her child again just to have him ripped away that she's crying into her long estranged husband's shoulder. He looks to Undyne, who still hasn't let go of Alphys, but now has his brother locked under an arm as well. He looks to his brother, and for once, Papyrus doesn't seem to mind Undyne's rough affection. He knows he doesn't have much longer with his friend. Sans looks to Frisk, who in childish naiveté is trying to jump up to Asriel, as though anything that they do now will help the situation. "I… won't… go…" Sans repeats.
Gaster looks taken aback. "I don't understand… this world, there's so much that isn't right about it. There's things missing. Things undone…"
"who cares!?" Sans asks. "so what if a little is missing? so what if not everything is perfect? does it really matter if in this old timeline i forget to calibrate a puzzle or two, or don't get to see everything in the world? is that worth resetting everything? worth erasing everyone's happiness here? …no. …no, it's not."
Gaster puzzles over that for a minute. "…Sans-" he finally starts, but Sans just shakes his head.
"don't…" he says quietly. "just… don't…"
Gaster is silent for a long while until a falling piece of the sky seems to startle him out of the temporary stupor. He looks up to Asriel and raises a hand, glowing with blue magic. A blue aura appears around Asriel and he's lowered, slowly to the ground. Gaster steps towards him, purposefully. Frisk runs between him and Asriel, holding their toy knife tightly. Gaster shakes his head and simply moves them aside with magic. He reaches Asriel and focuses, bringing his right hand up and quickly swiping it away from him.
The light surrounding Asriel dims considerably, and his limp form falls to the ground. Frisk goes to him quickly. A brilliant white heart rises over Asriel's body as it dissipates, shaking violently. …A soul? That shouldn't be possible.
With a cracking sound, it shatters in half. Frisk covers their mouth, in shock. But then the two pieces shake again, and they form back together, refusing to break completely. Asriel's body forms back around it and he turns on his side, coughing.
Frisk breathes a sigh of relief, then looks up at Gaster, who is holding a brilliant ball of light in his hands. "I caused the first fluctuations in time…" he says to them. "I created the cracks that led to these many timelines… I thought… perhaps, it was my duty to correct them. And I believe that still. But it wouldn't be me who truly pays the price then… would it?" He must know they can't understand, but he gestures to the barrier anyway. "A pretty big mistake to fix, isn't it?"
And he steps forward towards it. The area all around them is filled with a blinding light, the crashing stops, and the world falls still. Frisk is knocked back from the sheer force of the energy, and though they cover their eyes, they can hear beside them as Asriel stands and runs away.