The War of Humans and Monsters was a legendary clash that took place long before the events of Undertale.
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
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 magically sealed the monsters under Mount Ebott.
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.
- A monster with a human SOUL... A horrible beast with unfathomable power. The humans, afraid of our power, declared war on us. They attacked suddenly, and without mercy. - Ancient Glyphs in Waterfall
- Humans are unbelievably strong. It would take the SOUL of nearly every monster... ... just to equal the power of a single human SOUL. - Ancient Glyphs in Waterfall
- Not a single SOUL was taken, and countless monsters were turned to dust... - Ancient Glyphs in Waterfall
- Hey! That's the store of Gerson, The Hammer Of Justice! He's the toughest monster that ever lived! He fought in the War of Humans and Monsters.... And he survived! He's a true hero! - Undyne when calling her outside Gerson's store
- Seven of their greatest magicians sealed us underground with a magic spell. - Ancient Glyphs in Waterfall
- Trapped behind the barrier and fearful of further human attacks, we retreated. Far, far into the earth we walked, until we reached the cavern's end. This was our new home, which we named... "Home." - Book in Toriel's Home
- Fearing the humans no longer, we moved out of our old city, HOME. We braved harsh cold, damp swampland, and searing heat... Until we reached what we now call our capital. "NEW HOME." - Book in the Snowdin Library