The Unwanted Undead Adventurer - S01

While exploring a labyrinth, Rentt Faina, an adventurer with plenty of experience but little to show for it, is killed by a dragon after stumbling into its lair. Shockingly, he wakes up to find he has become a skeleton, the lowest class of undead monster. As Rentt fights off the other monsters in the labyrinth, he discovers that slaying them earns him experience toward higher stages of undead evolution, even ones with flesh. With this revelation, Rentt gains a new goal in his undeath: keep evolving in the hope of one day becoming a vampire, the highest form of undead, nearly indistinguishable from a human being. Though forced to hide his old identity and monstrous nature behind an unremovable mask, Rentt manages to return to some semblance of his former life. Given shelter by Lorraine Vivier, an elite researcher and retired adventurer, Rentt takes her surname and starts over from scratch as a rookie. Determined to chase his childhood dream of becoming a mithril-class adventurer, Rentt must be careful not to reveal his new bodily circumstances, lest the adventurers' guild try to exterminate the monster he has become.