There exists a society of people that worship the Sun as their God. Throughout the day they spend their time in awe and worship of their deity in the sky. As night comes, they retreat into their beds, ignoring the light of the moon.
Just as there exists a society that worships the Sun, so too exists a people that worship the Moon. Both societies try to disavow knowledge of the other, preferring to imagine themselves as the sole occupants of the world.
Each society prepares to conduct a ritual that names one of their people as their champion. The Champion of the Dawn, and the Champion of the Dusk.

As the ritual nears, two friends in the society of the Sun prepare for the possibility of being chosen. As the fabled day arrives, the ritual occurs. One of the friends is bathed in the holy light of the Sun God, declaring them as the champion that will wipe out the heretics of the Moon. Just as they believe the ritual to be over, an eclipse occurs, bathing the other friend in the silver light of the Moon, declaring them to be the Champion of the Dusk.
The Sun worshippers declare them as a heretic, to be killed, but they manage to escape to the society of the Moon worshippers. The Champion of the Dawn is given their first task, to hunt down and kill the heretic champion of the Moon...to kill their best friend...