The Strongest Sage with the Weakest Crest 05

The magician once known as a great sage has--in search of greater power--reincarnated himself far into the future. His new self has acquired the strongest crest for magical combat, but in an era where magic principles are all backwards, that crest is looked down on as the "crest of failure"... It turns out that evil demons were the ones who sowed disinformation and confusion about crests and magic, and now, those very demons seek to invade the capital and destroy all humans who know this secret. Matthias still doesn't have the powerful spells and physical abilities that he did prior to his reincarnation, but he uses his wits and creativity to take down the mighty demons!

Volume 5 of The Strongest Sage with the Weakest Crest manga series by Shinkoshoto.