Yori Shiro

The battle between Athos and Khama tribes lasts for hundreds years.
Khama dominate over the area with the springs of life granted by the gods, and Athos oppose their influence.
But there is a festival village where both tribes co-exist to perform breeding ritual.
Main character is in charge of the ritual. He does his instructor job despite the brink of war situation and the rivalry between the tribes.

[From vndbreview]