Manga Nozomanu Fushi No Boukensha

The main character was an eternal adventurer of copper rank from the border region, Lent. He was unlucky, since he met a powerful monster inside the maze, was defeated, and when he noticed, he had already become a bone man "Skeleton." Tormented, since he cannot enter the city in his current state, he recalls the existence of the evolution of monsters, in order to get a body with muscles, he defeats the monsters of the maze.