Fablehaven; Rise Of The Evening Star; Grip Of The Shadow Plague by Brandon Mull

Two young siblings uncover that their grandparents safeguard a hidden preserve full of magical creatures, and as they learn the rules and responsibilities they must face escalating threats: a secretive society seeking to unleash dark powers, dangerous artifacts and betrayals, and a mysterious shadow plague that corrupts beings. Forced to grow up quickly, they form alliances with guardians and fantastical allies, undertake perilous quests, and use courage and ingenuity to protect the preserve and stop forces that would plunge their world into darkness.