A brief description of the Nozomanu Fushi no Boukensha manga:
Lento Fayner is an adventurer who has been within the copper rank for about ten years. He’s been dreaming of promotion all his life. One day Lento encounters a secret passage in the maze, and despite the fact that it was logical to prepare from the beginning, and only then conquer the unexplored space, the man decides to take a risk. Well, the unknown truth opened up in all its glory: Lento ate the dragon. After that the young man wakes up, but now he looks like a skeleton. All that remains for the former adventurer is to develop as a undead. Will he like the new role?