It is known that Monsters previously lived with humans on the Surface, as shown in the game.
Several events happened on the Surface before Humans sealed Monsters Underground. A majority of the events have something to do with The Seven Wizards. Although there are no actual confirmed relevant events happening before the war between humans and monsters.
After monsters got sealed Underground, 5 of the Wizards disappeared under mysterious circumstances. Hundreds of guards tried to search for them but no trace of them was found. The Lightvale family decided to take over after the event. 
The Human Kingdom had declared war on the Monster Kingdom for an unknown reason. Casualties for both sides remain unknown, but according to Undertale, the humans had no casualties. It resulted in the Humans' victory. They sealed monsters into the Underground.
- Amber's name was never mentioned in any of the Human History Volumes.
- Kingdoms are named after soul traits because of the importance of magic in their society.