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" Long ago, two races ruled over Earth: HUMANS and MONSTERS.
One day, war broke out between the two races. After a long battle, the humans were victorious. They sealed the monsters underground with a magic spell.
– Undertale opening sequence
The War of Humans and Monsters was a legendary clash that took place long before the events of Undertale.
Monsters were too weak to put up a fight; the soul of nearly every monster combined is only just as strong as the power of one human SOUL. Countless monsters were massacred, without a single human SOUL taken. Few monsters survived, one of which was Gerson. Eventually, the monsters surrendered, and the humans emerged victorious. The humans gathered seven of their best magicians and sealed the monsters under Mount Ebott with a magic spell.
After the war, the monsters retreated into the deepest part of the Underground, which they called "Home." After a period had passed, fearing the humans no longer, they populated the rest of the Underground and made their capital in a place they called "New Home".
Monsters mostly blamed the war on humanity and took a vengeful look at the human race. This bitterness, especially after the death of Asriel and his adopted sibling, led King Asgore to declare war against humanity, installing a policy to kill all humans that fell into the Underground and eradicate humans once the seven SOULs needed to break the barrier are collected.