In the past long forgotten by most of the races, the demons waged an endless war against each other. The demons were a race with different tribes and clans. Most demi-humans with high amounts of mana or the so called intelligent monsters were considered demons by the other races.
The other races considered these demons to be a threat but at the same time, not a threat. They were a threat since every individual tribe of the demons were powerful beings. But at the same time, they weren't a threat since those powerful beings were too busy fighting against each other to even notice the other races.
...
The land were the demons lived consists of half the continent and in that land war was a common sight. To the demons, going to war was as natural as breathing and eating. It was a common occurrence to see tribes and clans having small skirmishes, here and there.