Revenants are similar to Zombies, in that they are both reanimated corpses. They are known the wither the area they inhabit, and they induce a paralyzing fear into anyone in the vicinity.
The Revenant that lived at Fablehaven guarded the Inverted Tower and could only be killed by removing the wooden nail lodged in the side of its neck. Seth and Coulter encountered this Revenant while in the grove at the Valley of the Four Hills, but Olloch the Glutton arrived and Coulter forced the Magic Cocoon into Seth's mouth. Seth later used Tanu's Courage Potion to counter the fear the Revenant induced, and was barely able to remove the nail before being rendered unconscious.
Calvin mentioned that Revenants wither the area they inhabit.