There are four major races in the demon world
Qinglong
white tiger
Suzaku
Xuanwu
The four major races have four sons
A future heir must be selected from among them, so the four clan leaders set up a competition arena to select one of their sons with the highest soul power and ability to rule the world. However, this competition soon evolved into 16 people from the four major races competing together.
This is how things happened...