Grip of the Shadow Plague (Fablehaven, #3) by Brandon Mull

Very strange things are afoot at Fablehaven. Someone or something has released a plague that transforms beings of light into creatures of darkness. Seth discovers the problem early, but as the infectious disease spreads, it becomes clear that the preserve cannot hold out for long. In dire need of help, the Sorensons question where to turn. The Sphinx has always given sound advice — but is he a traitor? Inside the Quiet Box, Vanessa might have information that could lead to a cure — but can she be trusted? Meanwhile, Kendra and members of the Knights of the Dawn must journey to a distant preserve and retrieve another hidden artifact. Will the Society of the Evening Star recover it first? Will the plague eclipse all light at Fablehaven?