Tada no Shikabane no You da to Iwarete Ikuseisou, Kizuitara Saikyou no Undead ni Natteta

Zion, an adventurer, entered a dungeon too dangerous for his skill level and died. He awoke centuries later as an undead being, continuing to move deeper into the dungeon. Over time, his undead form evolved repeatedly as he fought various monsters. When he eventually regained self-awareness, he had become the most powerful undead known as the "No Life King." The human race, upon learning of this new threat, feared for their survival, believing the world was doomed. However, Zion had no desire to destroy the world. The story follows the fear of humanity and the undead who does not seek to bring about their end.