Fablehaven: Grip of the Shadow Plague

I liked this book written by Brandon Mull. 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 in its infancy, 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 secretly 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.

This is the third book in the series.