This story starts 300 years after the event known as the merge.

The merge was an even when the two worlds known as Airous and Liongold fused to make Airengold.

Airous was a world without any magic and had no other truly intelligent species besides humans.

Airengold was a world of swords and magic

The merge also caused mana to get absorbed by the people and creatures of Airous giving them magic.

This caused the people of Airous to becomes stronger allowing them to get a foothold in Airengold.

All the kingdoms of Airous joined into an alliance and became the Airous empire so that their old world would be remembered.

Even before the merge humans roamed the land in Liongold.

At first the people of Airous and Liongold were suspicious of each other but since the people of Liongold didn't feel threatened by the people of Airous since they only just gained the ability to use magic. Since the people of Airous could fight back anyway the people of the two worlds quickly integrated. 

There was just one group that didn't join the alliance and that was the demon and monster territory.

They only had one wish and that was to kill.

Luckily the demons have a barrier surrounding their territory but slowly many demons have had the ability to leave the barrier but luckily it still stopped most of the demigods and pseudo god level demon from leaving. There leader who has remained unknown to this day is also said to be trapped and unable to leave.

The monsters don't have the intelligence to be a real threat. Monsters only have intelligence once they reach a certain level of strength or have special abilities.

There is one kingdom that is at the center of the world that is a neutral kingdom where all the races interact. This kingdom is ruled by a council made up a chosen representative of each race. This kingdom values neutrality when it comes to conflicts outside the kingdom and even lets demon in the city. Airengold's highest ranked magic academy is in the city with people from across the world going there to study. Even demons are allowed to study there but they are scorned by most because of the their innate nature. 

Demons are not born killers there are just ruled by a man that supposedly wants to create balance by destroying the world.