Long ago, there was a war between humans and monsters. After a long conflict, the humans were victorious, and imprisoned the Monsters under a one-way magical barrier - anyone can get in, no one can get out. The only way to break the barrier is to obtain seven human Souls.

Following the war, Asgore at first avoided any further hostilities with humanity. However, after the death of his son, Prince Asriel, Asgore declared war on humanity again, swearing to both break the barrier and destroy humanity itself. From then on, any human who fell into the Underground would be killed so their Soul could be used to break the barrier. Since then, six human children have fallen down via the ruins. All six were killed, and their souls taken.

Disgusted with Asgore's actions, Toriel went into self-imposed exile, leaving the kingdom entirely in his hands. She spends her days in the ruins, looking to adopt and protect any humans unfortunate enough to fall down into the Underground.