The Black Sage Weaves Shadows - The Saintly Deputy Was Banished as No Longer Having Any Use for Him, but the Curse [Dark Attribute] That Was Placed on Him Was an All-Powerful Cheat Magic

Kite, a court magician, has been putting up wards to protect the country in place of the saint. However, as soon as the crown prince's favored lady is chosen as the next saint, Kite is cursed to only be able to use dark magic and banished from the country. He is also forced to fight off a dragon that was accidentally summoned by the government... He has no choice but to start living as a hermit deep in the forest, along with the dragon he defeated... (Source: FireCross, translated)