Backstabbed in a Backwater Dungeon

DRAGONSLAYER! Sionne's cruel human experimentation and the Soul Dragon's all-you-can-eat spirit buffet come to an end when Light serves up just deserts! Then he receives shocking news--his sister Yume is alive! Can the former 15th-level twerp come to grips with his near five-digit power level?! Full Title: Backstabbed in a Backwater Dungeon: My Trusted Companions Tried to Kill Me, But Thanks to the Gift of an Unlimited Gacha I Got LVL 9999 Friends and Am Out For Revenge on my Former Party Members and the World (Manga)