Kuro no Kenja wa Kage wo Oru: Seijo Dairi wa Mou Youzumi dato Tsuihousareta ga, Kakerareta Noroi "Yami Zokusei" wa Bannou no Cheat Mahou Datta

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)