"i'm sorry g." Sans kneels next to his fallen opponent.
"Impossible! How did you-" Gaster groans in agony. "-defeat me? This should not have happened!"
"it happened bro. just let it go. you can come back with us. please." Gaster sighs.
"It is impossible. You were not meant to defeat me. The only way to escape would have been to kill you. I have failed." He sighs again. "You can't understand how this feels. This pain. This torment. This... emptyness. I just want to come back."
"destroying everything and killing everyone is not the right way g."
Gaster's face grows angry. "I'll use ANY means necessary!" A black tentacle appears behind Sans. It points straight towards his head, ready to strike.
999999
The tentacle fades into dust. Gaster, too, begins to fade. He now has two bones through his chest. He frowns.
"sorry." Sans says.
"It doesn't matter." Gaster laughs. "I'll only return to the void. I'm permanently bound to it. There's nothing that can be done. This defeat has probably severed any link I have with this world." He laughs even harder, and continues to do so until he disappears entirely.
"sorry dad." Sans repeats. Suddenly, the room begins to reform into the old castle. In a haze of numbers, everything begins to return. The monsters and ponies who had been captured by Gaster materialize in front of Sans and drop to the floor. They all awaken in confusion. Frisk hugs Toriel with tears in her eyes. So does Asgore, but one glare from Toriel prompts him to leave. Undyne and Alphys hug each other, then are swept into a hug by Mettaton. Papyrus runs over and lifts Sans into a hug, while crying.
"I KNEW YOU COULD DO IT BROTHER! I NEVER DOUBTED YOU FOR A SECOND." When Papyrus leaves to mingle, Sans notices Flowey sitting alone in the corner of the room. Frisk notices too, and the two of them walk over.
"What do you two want?" Flowey snaps at them.
"just wanted to say hi. hi." He then abruptly leaves.
"What about you?" He says to Frisk.
"How are you feeling? About all this?" She responds.
"Weird. I don't like it. If i could still SAVE and LOAD, I'd reset in a heartbeat." He thinks for a moment. "Say, why haven't you reset yet? You usually do by this point."
"This world is different. The Underground always opened up in the same place for so long. This is new. I'd never met Gaster like that. He was always... quiet, and left whenever I reached his room. This is the first time he's ever tried to escape."
"Please. If I had the six souls, I would have whooped him in a heartbeat."
"Plus, this world has pretty colorful ponies." Frisk smiles at Flowey, who gags in response. "Come on. It's time to head home." Frisk lifts Flowey's pot and joins the group. Twilight and Starlight lead them out of the forest. What greets them on that hill is a beautiful view of Ponyville.