Summoned into another world, Shimon awakens inside the deadly Twillight Dungeon, ruled by Astaroth, the [Undead King]. What was meant to be an RPG game becomes a fight for survival in a dark fantasy world where: Skills, magic, monsters, are other RPG-elements are real. LV Up Growth System for Skills/Spells is present. Death is permanent. With no escape in sight, Shimon makes a dangerous deal, to trade knowledge from his world in exchange of his life. As he trains and gets stronger, he encounters unexpected allies. A silent Golem and a mysterious mermaid, while preparing for the day he will have to challenge Astaroth to get his freedom. This is a dark isekai progression fantasy about survival, growth, and hope in a dungeon ruled by death. Can Shimon escape the Twillight Dungeon… or will this fantasy world break him first?