The Strongest Sage with the Weakest Crest 12

Is former Strongest Sage Matthias's fight to protect mankind finally over...or is it just beginning?! 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, Matthias and his party are in pursuit of a device that will lead them to a certain powerful demon! However, a nearby city is controlled by a group of demions-a demon subspecies that specializes in teamwork! They're using a factory to create a powerful bomb, so Matthias and pals have no choice but to put a stop to the scheme and slay all 32 demions! Perhaps a certain S-rank adventure will even show up again!

Volume 12 of The Strongest Sage with the Weakest Crest manga series by Shinkoshoto.