Grip of the Shadow Plague is the third book in the Fablehaven Series.
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 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? Find out in Fablehaven: Grip of the Shadow Plague.
Strange things are happening at Fablehaven. The book takes off right were the second book left off, during the same summer of their second year at Fablehaven. Kendra and Seth helped save Fablehaven from the Society of the Evening Star, but new troubles present themselves almost immediately.
Seth discovers that someone, or something, has released a plague throughout Fablehaven, before the others but as the disease spreads throughout the preserve, it is clear that they can no longer protect the preserve. Creatures of light are transformed into creatures of dark. As light creatures can enter most places throughout the preserve, the same rules apply to the more darkened form of them.
Sometime throughout the summer, Kendra is requested by the Captain of 'The Knights Of The Dawn' that she should be recruited, and for the first time, Kendra and Warren must visit another magical preserve called Lost Mesa, that is located in the central state of Arizona. They resign there, with Lieutenant Dougan Fisk, and dragon tamer Gavin Rose (whom Kendra has feelings for). Another hidden artifact must be recovered from Lost Mesa, before The Society unveils it first. However, Patton Burgess had already recovered the artifact and taken it to Fablehaven. Meanwhile, back at Fablehaven, Seth discovers that he has gained new abilities with destroying Fablehaven's revenant. He is able to shadow-walk, hear the voices of prisoners from the dungeon that dwell underneath the main house, and speak the language of many creatures such as imps, goblins, trolls, demons, and giants. (Kendra can recharge magical artifacts, see in the dark, control fairies, glow very brightly, and talk to many magical creatures like Seth.)
After coming back Kendra learns that Warren, Grandpa Sorenson, Grandma Sorenson, Dale, Tanu, and Coulter are shadows. However, she also learns that Seth has brought Patton to the future for three days using the Chronometer. Patton helps Seth and Kendra shut down the plague using a stone provided by the Fairy Queen and Lena dies when they shut down the plague.