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.
Human History Volume 1
“7 of the most powerful human wizards vanished the monsters from the surface. Using all their strength they created what we call ‘The Barrier’. It’s what keeps the monsters in the underground away from the humans preventing them from harming us. The wizards formed the Council and ruled over humanity as a democratic entity. But as years passed by, the wizards got older and older. Eventually they passed away. When it got to the point where only 2 of the original wizards were left, the youngest of the group, siblings, they had to face upon their people’s demands. Their kingdom demanded a single ruler. It was either one or the other. The kingdom wanted a show off of the wizards’ magic powers to choose their leader. In the end they choos–”
Gaster: Oh… the rest of the page is unreadable, guess I’ll have to skip a few pages, these books are ancient indeed no wonder it’s even missing a few pages… hm?…knocking?, who could be visiting me now, *sigh* guess I’ll continue the reading later on. I have to work on the Core’s expansion anyways.
Human History Volume 3
”The following are all the extracts that can be translated so far from old books that our ancestors tried to keep generation after generation.”
"After the War" The new King, Roy Goldburn, was out of his castle most of his time. Looking for monsters that had escaped and ignored his orders. Any monster found was sent to jail or forced to work under the kingdom’s watch. While some thought the measure was extreme, a fair part of the population was scared of the possibility of being killed by a monster and have their soul taken and preferred to keep the few monsters left put there under the humans watch, than to leave them free roaming around our lands. The king would often ask for the wizards from other kingdoms to help him on his journeys. While it is said that the king was just looking for normal monsters, there are rumors that say he was looking for one monster specifically. One that had already, multiple human souls in his power. After 2 months, the king ordered to call all the wizards to his castle for a secret meeting. What was said and discussed is still a mystery to everyone, but we suspect that whatever the king was looking for, he had found its location, which required all help possible. He asked the wizard of perseverance, integrity, kindness, and patience to go with him on a 3 days mission; leaving the young wizards of Determination and Bravery to be in charge of all the kingdom’s duties for these 3 days. What humanity never expected to happen, was that all 5 wizards, disappeared in said mission. From all corners of the continent, hundreds of guards from all kingdoms were sent to look for our missing leaders, but despite everyone’s effort, they were never found. Not even a trace. The king never mentioned where he was going, or what was he after. All humans were desperate and didn’t know what would happen from that point and on. It was then when the Lightvale family decided to step in and take control of everything, before humanity sunk into complete chaos. Everyone thought it was going to be too much work for the young siblings; to our surprise, they proved us wrong. Implementing innovating systems that included the participation of all kingdoms, it seemed like humankind would overcome this tragedy.
“The second tragedy" But then, after 10 years of peace, the two siblings, both powerful wizards, who had agreed on everything during their shared ruling of the kingdom, began to argue over their opinions on monsterkind. Copper Lightvale, our Determination wizard expressed his desires to free monsters that were sealed underground. Having traveled all corners of our continent he had seen how monster-kind could live along side with humans in complete peace, helping each other as equals. Agate Lightvale, his twin sister, insisted in believing on our old King’s warnings, and keep monsters apart from humankind to ensure the race’s safety, and to keep another tragedy from happening. Both siblings never got to an agreement and began to demand to have the final word on the matter. Their people began to demand for one ruler and settle things once and for all. Agate came up with the idea and challenged her brother to a magic battle and decide who would have the right to decide over the other. Even though it wasn’t a battle to the death, Agate behaved like it was one. But, her brother’s Determination had no limits, leading him to victory. Something in Agate changed that day. We barely saw her outside her castle since the battle took place. And after a week, she left. Her brother was extremely concerned. He sent out guards to look for her day after day. Stress began to overwhelm him as weeks went by, having to take care of all of Agate’s duties, raising the future princess, looking for his missing sister and looking for a way to open the barrier. His smile slowly faded away and even the kingdom began to worry about his health. It was then when she came back. Her eyes were not the same. In fact, when she first appeared no one recognized her. Copper welcomed her sister with open arms, having even a party to celebrate Agate’s return. The rumors say that the guards standing near her, couldn’t help but feel fear towards her. It wasn’t necessary for her to make eye contact, the fact of standing near our queen caused us to feel shivers down our spine. A few days passed until Agate asked her brother for a second battle. Copper denied his sister’s demand, but apparently, she forced him to accept it. A dark feeling came upon the entire kingdom. Agate was no longer the sweet queen we all once knew, her sole purpose was to take revenge upon his brother. Copper ordered the guards to evacuate the kingdom and nearby areas, he felt Agate’s evil intentions and that she wouldn’t hold back. The people that were present and watched the fight, say that Copper held himself together most of the battle. But that Agate did something that caused her brother to lose his Determination rather than overwhelm his power. It was there when she killed him. People saw terrified as Agate dealt a fatal blow on her brother’s chest, shattering his visible soul in her hands. Everyone abandoned the kingdom and ran to nearby countries, warning everyone of what their queen had done. Everyone got prepared to fight against her, but after the battle, she was never seen again.
"Rebirth” Humankind fell into chaos, with no rulers, people felt lost. Considering humans’ natural cruelty and desire for power, we were terrified. Somehow, miraculously, and after so many tragedies, it felt like everyone agreed to try and do things the right way for once. Thanks to the noble family from the kingdom that belonged to the kindness wizard, they decided to allow monsters’ intervention in political and economic issues. Both races cooperating together allowed their kingdom to stabilize itself and reach peace. the other countries slowly began to adapt the kindness’ kingdom’s practices, in fact, decades passed before another kingdom finally accepted monsterkind among humans again.
- 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. 
- ↑ "The new king, Roy Goldburn"
- ↑ "It was then when the Lightvale family decided to step in and take control of everything,"
- ↑ Kingdoms are named after soul traits because of the importance of magic in their society (Link)