Nozomanu Fushi no Boukensha

The protagonist was an eternal copper rank adventurer of the frontier area, Lent. With his bad luck, he meets a powerful monster inside of the labyrinth and is defeated, before turning into a bone man《Skeleton》.
Agonizing because he can't go to town in his current condition, he recalls the existence of monster evolution, and, with a goal to gain a body with muscle, he goes to defeat the monsters of the labyrinth.