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

Zion, a solo adventurer, recklessly entered a dungeon beyond his ability. Alas, that was the last thing he did. Wandering inside a dungeon for hundreds of years, as an undead, he kept advancing into the dungeon. Slaying monster left and right until his undead body underwent "evolution" many times ov