Taizai no Majo -A Seacret Sin-

Long ago, four witches called the Witches of Deadly Sin ruled an island country in the air, that is the city of the sky. 1000 years later, witches are severely discriminated against, and anyone found guilty of being a witch is imprisoned and sentenced to death. The main character, a Knight of the Kingdom of Arial, Hart-Duraflame, is commanded to hunt witches.