Wraiths are ethereal dark beings, similar to ghosts. In book four, Grandpa Sorenson discusses with Seth the wraiths imprisoned in the Hall of Dread. He also threatens the Maddox stingbulb with a close encounter with a wraith.
Seth momentarily places three wraiths around Fablehaven to prevent the centaurs from taking all of the Secret Preserve as their own after freeing them from the Dungeon after they had agreed to his terms. He has since taken them away. Seth promised the singing sisters a wraith to do with as they please.
Powers and Abilities
Wraiths are stronger at night. Wraiths can take on a hundred centaurs at inside, underground or in a man-made structure but in sunlight would fall.
When visiting The Hall of Dread to find information hidden by Patton, Seth hears voices whispering in the dungeon, and notes he hears them. The voices shut up, and one starts to persuade Seth into opening the prison, saying he would serve him for a thousand years. Seth tells he has to think about it, and leaves the prison, with the wraith saying he would wait.
Later, in Wyrmroost, at the Blackwell, Seth hears voices in it, asking him to open the prison, saying they would obey his every command. Later, when Agad finds Seth, he hints that the creatures in the dungeon are very powerful undead, possibly wraiths, and that the Black well is possibly the most dangerous place in the whole sanctuary.
After the fall of Fablehaven, Patton tells Seth to start hiring wraiths or other dark creatures at his service to guard certain places at Fablehaven, such as the house, old manor and the stables. Seth hires the wraith from his last encounter, but almost falls into a trap set by it and its two brothers.
After the centaurs try to take the Old Manor and the house, the wraiths beat them back and possibly kill some, as Seth promises them every creature that comes near the house as a reward for their services. This results into the centaurs proclaiming war on Seth. Seth also gets an idea to built a large army out of wraiths, zombies and trolls, and asks the oldest of the brothers what chance would he have against the Demon King, to which the wraith simply replies "None".
Later, Seth promises three things to the singing sisters: Vasilis the sword of light and darkness, a wraith to do their bidding, and one thing they wish. It is also revealed Sphinx has a company of wraiths with which he hunts down Warren. The same wraiths save Warren from a Manticore.