Sans leads the monsters towards Twilight's castle. As they run towards the castle, strings of numbers float vertically out of he ground. Trees and buildings begin to fade away the same way Toriel did. Once they reach the castle, the front door fades away. In the throne room, Twilight is attempting to restrain the numbers. Spike is swatting at them with a broom. Upon seeing the monsters, Twilight teleports over to them.
"Do you guys know what's happening? Where's Toriel?" She says frantically. Asgore fights back tears.
"Toriel is gone. She was taken by the glitches."
"i know what's happening." Sans says. "it's an old pal of mine: w.d. gaster."
"W.D. who?" They all say in unison.
"the old royal scientist, before alphys. he designed and issued the construction of the core."
Recollection flashes in Asgore's eyes. "Aw yes. Dr. Gaster. I had forgotten about him and his... unfortunate fate."
"everyone forgot he existed. except me."
"Wait, 'unfortunate fate'?" Twilight asks.
"As Sans already mentioned, Dr. Gaster created the Core." Alphys answers. "Using ice blocks supplied from Snowdin, the Core created the power for the entire underground. One day, Gaster was looking over his creation when someone pushed him. He fell into the heart of the Core, and... died."
"that's not how the story ends."Sans sighs. "after he fell into the core, he didn't die. he was technically erased from existence, but he was actually banished to what is called the void. everybody completely forgot he existed. that is, except me and his followers."
"His followers? Who are they Sans?" Asgore asks.
"you wouldn't know them. a real mysterious bunch. all grey and black. they're very elusive. one of 'em even looks monster kid. but now back to G. i don't know how he's able to leave the void. i would guess he's making his way around town creating the glitches, but he can't fully materialize because of being in the void and can only show up as a fleeting shadow. since he shouldn't exist, his touch is causing everything to be erased like he was. there's a chance that stopping him and banishing him for good would reset the damage done, but we won't know until then."
"How is he able to turn people into glitches like that?" Asgore asks.
"More like how can he turn EVERYTHING into glitches like that." Undyne adds.
"probably some weird void powers."
"MISS TORIEL SAID HE WAS COMPLETELY WHITE. IS HE A SKELETON, SANS? LIKE US?" Papyrus asks.
"i dunno pap. maybe. what we need to do is gather some people together and find the g-man."
Asgore steps forward. "I will go. As king of the monsters, it is my responsibility to stop one of my own from harming others."
"I'll stay here." Alphys says. "I can communicate with you through your phones. I can update you on what's happening here." Twilight steps forward.
"I'll go too. I can't stand by and let this happen."
"No Twilight." Starlight enters the room. "Stay here and keep the peace. They'd be more likely to listen to you than me. I'll go."
Twilight tries to counter with, "But..." but a firm look from Starlight makes her back down.
"I'll go." The voice of a new visitor echoes through the room.
"Bon Bon?" Twilight and Starlight both say in unison.
"I'm a former secret agent, remember? I'll be a valuable asset to the team.
"I'll go too!" Undyne announces. "It's been too long since I got to kick some-"
"I WANT TO GO TOO!" Papyrus interrupts.
"no pap. it's dangerous." Sans responds.
"I AM NOT AFRAID OF DANGER! NYEH HEH HEH!"
"this is a whole other level of danger. i don't want to lose you. if you disappeared, i'm... not even sure beating gaster will bring anything back."
"BUT SANS..."
"Sans is right Papyrus." Asgore cuts in. "It is very dangerous. The more people who come with us, the more chances of someone... not coming back. We can't have that." Papyrus frowns and looks at Sans, then solemnly walks out of the room.