The Oath of Dawn (Gesh)

Erin is a land targeted by the evil god Fior. Cool, a knight in training, and Sei, a sorceress in training, belong to the Ogma Knights and work diligently every day as a pair to defeat monsters. Eventually, their strength is recognized and they are given their first mission, but...? A Celtic action fantasy with a story by Mitsuru Yuki, author of the "Shonen Onmyoji" series.