The War of Humans and Monsters

The war was a legendary clash between humans and monsters from the distant past. Humans won the engagement due to their more powerful souls, and physical nature. This allowed the humans to banish all monsters into the Underground, where they placed a Barrier that separates the Surface and the Underground.

In the Underground
Monsters largely blame the war on humanity, and take a vengeful look on humans as a species, although specific monsters befriend the main character, even after realizing they are a human. This bitterness, especially after the death of the First Human, and Asriel, led Lord Asgore to kill all humans that fell into the Underground, so that he could collect the Seven Souls required to destroy the Barrier, so that he and all other monsters could return to the Surface.