Strongest Sage With The Weakest Crest (Manga) Vol 15

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! Square Enix Manga
(W) Shinkoshoto (A) Kansho & Hyoko, Huuka Kazabana