Fablehaven

Fablehaven is a 2006 fantasy novel written by Brandon Mull. The book centers on two siblings, Kendra and Seth Sorenson, who visit their grandfather's mansion, which unbeknownst to most of the world, is a reserve for mystical creatures. The grandfather and the caretakers are eventually kidnapped by imps, forcing the siblings to find and rescue them while a witch named Muriel plans to unleash a powerful demon named Bahumat.