Rentt has been stuck as a bronze-rank adventurer for ten years. One day, while exploring a well-traveled dungeon, he happens across an undiscovered passageway. Venturing down it, he is immediately attacked and killed by a great dragon. Somehow, he wakes up a short time later, now an undead skeleton but still possessing his memories and abilities. Recalling the process by which monsters can get stronger forms, he resolves to do just that in the hopes of taking a form that can pass for human.
Contains the following tropes:
- Adventure Guild: The adventurer's guild is part of a larger organization which manages the assignment of missions ranging from simple errands to dangerous monster hunts. Ranks are named after the metal of the membership emblem, with mithril being the highest rank.
- Bait-and-Switch: When Rentt is offered his choice of rewards from a warehouse, he becomes fixated on a vial of vampire blood. The scene then switches to him and Lorraine examining the item he selected: A remote control zeppelin he'd been looking at earlier. Subverted when Lorraine ignores the distraction to discuss the vial which Rentt had also received as an apology.
- The Beastmaster: The descendants of King Felt, who are actually the villagers of Hathara, have the power to command monsters to do their bidding.
- Clingy Costume: Among the clothes Rino buys for Rentt is a metal mask so he can hide his undead features. Only after putting it on does Rentt realize it's a magical artifact which bonds to him and cannot be removed. He can dematerialize the upper or lower half as desired, though not the entire thing at once.
- Dem Bones: Animate skeletons are monsters found in dungeons. Rentt turns into one after his death.
- Evolution Power-Up: Monsters have the ability to undergo "existential evolution", transforming into a more powerful type of monster. The exact mechanics are a subject of Lorraine's research and seem to have different conditions based on the monsters. Rentt gains access to this ability as well when he becomes undead. He evolves from a skeleton into a ghoul after killing several monsters, a a thrall after ingesting human flesh and blood, and a lesser vampire after drinking vampire blood.
- First-Episode Resurrection: The story starts with the protagonist Rentt dying when he gets eaten by a dragon and then he comes back to life as an undead skeleton.
- Freaky Fashion, Mild Mind: Rentt's disguise is an intimidate black trenchcoat or robe with a skull-shaped mask which initially unnerves some people, but Rentt himself in a good-hearted man.
- Gashadokuro: Rentt fights a giant skeleton when he goes dungeon diving with Loris.
- Hedge Maze: The noble Latour family maintains a hedge maze as the entrance to their estate and rewards anyone who successfully passes through it their choice of a magical artifact. The trick to the maze is that it's filled with hidden teleport spots that make mapping it normally impossible.
- Hide Your Otherness: Rentt has to hide his transformation into an undead because the church would view him as an abomination to be purged and many people would likely agree.
- Jack of All Stats: Humans have the ability to utilize three types of magial energy: Spirit, mana, and divinity. Individuals have different affinities for these energy, with most being able to wield only one or two of them. Rentt is rare in that he can channel all three energies, though his skill with each individual was unexceptional prior to his death.
- Job Title: The protagonist Rentt is the titular undead adventurer.
- Lost Technology: Artifacts recovered from labyrinths are highly advanced magical items which modern societies cannot reproduce, though some technology can partially recreate their effects.
- Oblivious to Love: Rentt remains unaware that any women, such as his friend Lorraine, might have feelings for him.
- Our Dragons Are Different: Dragons are the most powerful and rare monster in the world, with some legends claiming they are in fact divine beings. The dragon which kills Rentt at the start of the story resembles a vast frog or snake with human arms, angelic wings, and a halo.
- Our Vampires Are Different: Vampires are a rare and powerful form of undead known for drinking blood to absorb their victim's essence. There exist several classes of full vampires while Thralls are said to be servants they create from humans after drinking their blood. Legends claim that drinking a vampire's blood will cause a human to become a vampire themselves, but in reality it either kills the human or drives them insane. Rennt evolves from a thrall into a lower class vampire by drinking a vial of their blood.
- Secret-Keeper: Lorraine agrees to keep Rentt's changed nature a secret and help him as she can. Sheila later convinces Rennt to explain why he's hiding his identity and agrees to keep his secret as well.
- Secret Legacy: All the villagers of Hathara Village, Rentt's hometown, are descendants of King Felt. Their ancestors went into hiding because of civil wars between their clans broke out over the valuable magic powers inherited from the king.
- Swallowed Whole: Rentt dies when he gets Eaten Alive by a dragon in one bite. Later on, the same thing happens to him again with another dragon, but this time he doesn't die because he's already an undead.
- Sword Plant: The first novel cover has Rentt striking a badass pose with his sword planted.
- Teleportation: Teleportation magic circles can be found in dungeons.
- Underground City: The Underground City of the Virtuous King Felt on the 60th floor of the Ancient Labyrinth of Insects. The Hathara Village has a secret portal that connects to it.
