Grip of the Shadow Plague
The Shadow Plague was created when Navarog the dragon gave the nail to Kurisock, who binded it with what was once the tree of Ephira. The affliction affected only light creatures, changing them dark. When it affected humans, however, they merely changed to invisible shadows, who could not be seen by anyone except Seth. Light creatures were able to change into dark ones, but they had no way of changing dark creatures back into light ones.
If a human was bitten, they would have a shadowy area spreading from the place that they were bitten but to most eyes it would look invisible. If a creature was bitten, they would have a maroon wound appear at the site of the wound. The wounds would spread slowly or quickly, depending on the number of bites, and eventually the fallen creature would change as the plague would take effect. Most of the denizens of Fablehaven would have new powers (beyond the ability to spread the plague) and would generally be larger, stronger, and more prone to anger.