Argonaut: Dungeon ni Deai wo Motomeru no wa Machigatteiru Darou ka - Eiyuutan

This is a tale of a distant past, long before the gods' blessing came to the land. It is the legend of an adventurer still told to this very day. He was by no means history's strongest or most inspiring, so how exactly did Argonaut reach such famed heights? The perfect opportunity to claim his rightful title presents itself when the king summons the land's mightiest warriors for an all-important quest, but the young fool comes to realize he may have bitten off more than he can chew... The curtain rises on a comedy that echoes through the ages!

(Source: Yen Press)