The Strongest Sage with the Weakest Crest Vol. 5 (manga)

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 it’s time to use his wits and creativity to take down the mighty demons!