At the earliest, the black-haired world and the red-haired world were actually the same world, with no difference in blood.
But - hundreds of years ago, a horrific genetic experiment tore apart the population: In order to create more powerful human weapons, scientists designed the "red-hair factor", which allowed some people to gain supernormal strength, but also lost the ability to control their emotions, becoming manic, extreme, and bloodthirsty.
As a result, the experiment went out of control. The red-haired tribe began to riot. In order to survive, ordinary people (the black-haired tribe) built a barrier to block the territory of the red-haired tribe and expel them from the normal world. And they revised history to characterize the "red-haired riots" as an inherently evil racial rebellion, covering up the truth that the black-haired people themselves caused all this.
The red-haired world has since become an abandoned "wasteland".
The black-haired world, on the surface, is stable, but deep down it is full of rejection and fear of "others".
Intention: Even if the world is broken thousands of times, I am still willing to walk through it all over again for you.