Manga The Unwanted Immortal Adventurer / Nozomanu Fushi no Boukensha

Lento Fayner has been an adventurer for 10 years, killing low-level monsters in labyrinths for a living. It’s a job he’s stuck with since he’s only Bronze rank, but he has dreams of ascending to the legendary Mithral rank.

One unfortunate day, he discovers a new part in the labyrinth and comes across the godlike powerful dragon. When he is eaten, he’s sure that he has died...except when he opens his eyes, he’s become a skeleton monster! Counting on a monster’s ability of “existential evolution” -- gaining power by defeating other monsters -- Lento sets out to get stronger and regain his life!