Kuro no Kenja ha Kage wo Oru

Kaito was a court magician who created barriers to protect his country on behalf of the Saint. However, as soon as the Crown Prince's favorite duke's daughter was appointed as the next Saint, Kaito was cursed to only be able to use dark magic and was banished from the country. What's more, the country forced him to deal with a dragon that was mistakenly summoned... so he had no choice but to start living as a pharmacist in the depths of the forest with the dragon.