The Strongest Sage with the Weakest Crest 15 (manga)

Matthias makes a shocking discovery while investigating the imperial waterworks. Grevil, a wise king from ancient times—and an old friend of Matty’s—is somehow alive and working with demons! Grevil gives Matthias and his party three months to prepare for a battle to decide the fate of humanity. After extensive training in a dungeon, Matthias and his party decide to confront Grevil, who greets them with a grand public spectacle?!