Shikkakumon no Saikyou Kenja

The sage Gaius reaches the limits of the magical crest he was born with and chooses reincarnation in hopes of receiving a mark better suited to close combat. Thousands of years later he awakens as Mathias Hildesheimer with exactly the crest he wanted, only to discover that this era calls it the Crest of Failure. Magic has declined so badly that basic techniques from his time look miraculous, while misinformation has become accepted teaching. Mathias enters the royal academy and overturns its standards with ease, but proving his strength is not the real problem. Demons have benefited from humanity's ignorance, and he must restore lost knowledge before that decline becomes the weapon that defeats mankind.