Briefly about Nozomanu Fushi no Boukensha Manga:
A frontier area adventurer by the name of Lent, Rentt Faina was an immortal copper rank. He confronts a formidable monster inside the labyrinth, falls prey to it, and discovers that he has already transformed into Skeleton, a bone man. He recalls the existence of monster development as he agonizes over the fact that he cannot enter the town in his present state. With the aim of gaining a body with muscle, he overcomes the labyrinth monsters.
One day an adventurer of the copper rank, Rent stumbled upon an unknown passage in a labyrinth, and though everything said he shouldn’t go there, he went anyway. There his life ended. But he didn’t die; he woke up not as a man but as a skeleton. Now all he could do was evolve as an undead.
