It took a long while for Asgore to calm Asriel enough to stop hounding Sans with questions. Chara was easier to persuade, being the quieter of the two. Finally, the three were sitting down at a table in the study, snacks and juice out for them all with paper and pencils out for drawing and colouring to their heart's content.

Gaster, Asgore and Toriel were at another table, and Gaster kept glancing over at the children to make sure Sans was alright. Sans was also continuing to glance over at the adults. He'd not considered himself a talkative child, but it was one thing to not want to talk to others, and another to not be able to.

"So…" Asgore said, quietly, placing his teacup on its saucer. "Tell us about Sans."

Gaster looked back at the king and queen. "Well… when I got to the crater, he… was already falling." The skeleton told them. "I was too late to save him, though."

"Save him…. Was he… human?" Toriel asked.

Gaster nodded, trying to be sensible about this. "There is something in that cavern. It's like… a bubble. It…. did something to him, after he died."
"A bubble?" Toriel wondered.

"It was a weird shimmer." Gaster said. "Like how soap bubbles are sort of… rainbow?"

Asgore frowned. "I… don't understand, Gaster. What did it do?"
Gaster seemed a little upset. "Well… it looked like it was…. Sucking something out of his soul. Magic, maybe." He explained. "But it wriggled free, and then went back to him. And he turned into… what he is now."

"But that… can't be." Asgore responded, clearly bothered by the thought of this. "Humans can't…. Become monsters."
Gaster frowned. "Well, clearly they can." He said. "Because it's happened twice."

Toriel and Asgore exchanged a glance. "Twice…. You don't mean to say…."
"I appeared out of thin air, otherwise." He finished.

Asgore seemed worried. "Gaster… the only thing I can think of that even remotely resembles what you saw… is the Barrier." He said. Sometimes it was nice to have yourself proven right, but Gaster wasn't sure if this was one of those times.

"You've never really explained the Barrier to me." The skeleton said.

Asgore sighed, and sipped at his tea.

"Gaster…" Toriel said, voice wobbling a little. "We…. Monster-Kind fought against the humans in a war." She explained. "And, we lost. The humans forced us down here, and cast a spell over the mountain. There is a Barrier that seals us in…. Things can pass through it to come in… but nothing can get out… unless their soul is more powerful than the Barrier is."

That…. wasn't what Gaster was expecting. "But…. Monsters have so much magic…"
"That's the problem." Toriel continued, while Asgore was still awkwardly sipping his tea. "We have so much magic in our bodies, that we don't have as much in our souls. Humans on the other hand, seem to be the other way around: magic in their souls but not in their bodies."

Gaster frowned, and put a hand on his chest in thought.

"I don't recommend trying it, Gaster." Asgore said, seeming to hear Gaster's thoughts. "The war is still too new. You wouldn't be treated kindly on the other side, even if you could get through."

"So then what was it doing to Sans? To me?"
"I don't have an answer, Gaster."

It was aggravating. Not that it was really Asgore's fault… the Barrier was clearly a form of magic that was foreign to them, although Gaster was surprised to hear that the Barrier had been cast by humans, when it had been his impression that magic was for stage shows and nothing to be taken seriously.

"...This is going to keep happening." Gaster predicted. "I'm of the opinion that if I hadn't been able to catch Chara when they fell, they would have become a monster, too."

"I don't know what you would have us do, Gaster. We can't very well fill in the crater." Asgore said.

Well, they could, as far as the Barrier, at least. But that was a waste of time, given that the drop would still be enough to kill someone fairly handily. Gaster lay his head on the tabletop beside his teacup. He was tired.

Asgore pet his friend's back. "Hey, come on now…. I'm sure we can get to the bottom of it."