We never really get to learn just HOW monsters came to the point of being sealed under the mountain. I thought I'd take a shot at writing it. So this is my pre-canon speculation on how it went down. Left vague since we have no idea who might've made the call to seal the monsters away.
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The flat of the blade was cold against her throat. A steadying reminder to be still. A reminder this was a last chance.
"Please." Asgore's voice broke the silence, a plea- something Toriel had grown used to over the past year. "Please, we mean you no harm. We wish to broker peace."
"Peace?" The human king blustered back, light colored eyes drifting across the royal monsters before him. His knights creating a shield between monster and man. "Why would I want to broker peace? We're nearly rid of you lot."
Anger rolled through her, making her teeth ache and fingers itch to pull fire to the surface, to watch this place burn. Logic always won, even if anger was an alluring feeling. "Because, we wish to stop this war. To be out of your sights."
The human king hummed at that, "What can you offer me? How much more can you be out of my sight than dead?"
"Exile." The word tasted like ash in her mouth. She could feel Asgore behind her, the familiar presence offering her some comfort in the moment. "We can all leave your lands, go far away. Be out of sight and mind."
"Tempting."
"Please, we beg of you." Asgore's hand settled against her back, protective but gentle. Toriel just wished she could turn to hold him. She could only imagine his face. The agony he had hidden away for so long finally revealed.
"Well, if you beg, who am I to say no?" He laughed, the sound was sickening. "What if, we sealed you under the great mountain?"
"A mountain?" Asgore's fingers tightened at the ratty fabric of her robes. The shift of metal behind her was enough to tell Toriel he was being warned against any other movements.
"Mount Ebott." She answered softly back, all the anger fading to resignation. "How would we be sealed?"
"With magic. Only fitting after all." The king stroked his mustache thoughtfully, before motioning his attendant forward. "Bring me the high mage. We have much to discuss."
Toriel spoke, catching the king's eye with her gaze. "Will you honor a treaty? One where we agree to be sealed? No more death of our people in exchange for exile?" Tension radiated off of Asgore behind her. She knew, so easily and so immediately he hated all of this. As much hate as the gentlest monster she ever knew could muster.
He frowned at her tightly but nodded, "Yes. I am not one for breaking treaties."
Toriel wanted to laugh, oh how she wanted to laugh. What of the treaty before? The treaty of peace between humans and monsters? The promise of safety and friendship?
"You are an honorable sort. So I believe you." She would sooner believe humans could fly than King Dawkins was honorable in any regard. Whilst others were scared of them, he merely thought they took up lands he wished to have.
The man grinned, "I knew some of you beasts were smart." His gaze drifted to Asgore. "You've picked a good wife. Sensible enough."
"Yes, I have." Everything in Asgore's voice was emotionless, forced to coldness. It barely sounded like her husband.
The king turned from them, speaking to his attendant now. Catching the young man by the elbow unkindly. "Bring me some paper, we'll work on a treaty. The mages can wait for a moment."
Toriel simply stayed patient and quiet as she and Asgore signed their people's futures away. She would cry later, cry with Asgore. The two had finally saved their people, but at a terrible cost.