The Starfall Dungeon. The Myriad Realms, #1

Daniel Cole died saving visitors inside a collapsing museum. He woke without a body, trapped inside an ancient Dungeon Core that should have possessed a fairy attendant, safety restrictions, and a sensible beginner's guide. Instead, Daniel received administrator access, a nearly empty mana reserve, and an urgent command to anchor beneath the ruins. He refused and set out on his journey home.