The lore for Peria Chronicles is still a bit unclear. The developers did put a post about it very recently, however it was not worded clearly. This is what we got out of it with some speculation:

There was once 2 realms. Peria, the realm of the humans and Araca, the realm of chaos controlled by the god of chaos Akasha. Something happened and caused the realms to crash into each other letting the inhabitants of Araca enter Peria.

The inhabitants are called the Kirana. These beings had powers unknown to humans and could easily wipe out the human species. However, they could not live long in Peria on their own. Their very life force is drained away upon entering Peria. The humans pose no threat to the Kirana and decided that they could coexist.

Now some kirana befriend the humans, creating contracts that allow them to freely enter Peria under the limitations of being close to their human masters. Through these contracts the humans gained newfound powers.

The humans and Kirana coexisted well, and some Kirana's and humans gained a special contract. These Kirana are called patron Kirana and have the ability to merge with humans giving them even more power.

Together the humans and Kirana coexist to fight against the evil god of chaos, Akasha.