The Unwanted Undead Adventurer (Manga)

It's been ten years since Rentt Faina, a down-on-his luck Bronze-class adventurer, set his eyes on becoming Mithril-class. Unfortunately, Rentt finds himself helpless when confronted by a legendary Dragon in the Labyrinth of the Moon's Reflection... and is summarily eaten. But Rentt has miraculously reawakened... as a Skeleton?!