Teogonia: Volume 3 by Tsukasa Tanimai

The demi-human macaques that rule the great forest have brought disaster upon their species. A profane creature, known as a diabo, drains power from their gods and threatens to turn their home into a barren wasteland.

None of this would normally concern a human boy like Kai, but the macaques have named him as their protector, and his god insists that the diabo must be slain. Apparently, their diabo is his problem now.

Meanwhile, the guardian bearers of the borderlands are making preparations to attend the winter solstice banquet in the provincial capital. There they will drink, feast, and fight, thereby strengthening their ties. As Lag’s most promising young soldier, Kai has been invited to attend and witness this gathering of powerful warriors for himself.