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 over. By the time he regained his ego, he had...