Furuki Okite no Mahou Kishi

A long, long time ago, in the era of legends, there was a knight known as Sir Sid the Barbarian who pledged his sword to the Holy King Arthur. Unparalleled in strength, he was the greatest magical knight of his time, but also a disgrace to the name of chivalry. His evil deeds were countless, and eventually he met his end at the hands of his own king... or so the story goes. Now a thousand years have passed, and Sid is summoned from his eternal slumber by Alvin, a descendant of Arthur and a prince who knows the truth about Sir Sid the Lightning Knight. Honoring his ancient pact, Sid rises from the grave to help Alvin save the kingdom from impending peril and become the new instructor to Alvin's class of fellow aspiring knights. As danger lurks in the shadows, Sid must train this class of underdogs to rely on more than just their magical fairy swords and teach them the old ways of the magic knights.

(Source: J-Novel Club)