Fablehaven (Book 2): Rise of the Evening Star, Brandon Mull

At the end of the school year Kendra and her brother Seth, find themselves racing back to Fablehaven, a refuge for mythical and magical creatures. Grandpa Sorensen invites three specialists - a potions master, a collector of magical relics, and a trapper of mystical creatures - to help protect the property from the Society of the Evening Star, an ancient organization determined to steal a hidden artifact of great power. Time is running out. If the artifact falls into the wrong hands, it could mean the downfall of other preserves and possibly the world.