A wraith is a type of undead that is created when a person, a creature or another undead places their soul into an object, thus becoming a mind without a soul and without a corporeal form. In terms of powers, memories and abilities, they retain whatever they had prior to becoming wraiths. They are subject to damage to whatever body or object they are currently possessing and can feel pain, although the only way to permamently destroy a wraith is to destroy the soul object, which will grant it mortality and make it killable.
A wraith can possess any object or DEAD body they desire. Any living body with a soul in it will immediately reject the wraith. Also worth noting is that a wraith possessing an object with an owner becomes immediately bound to said owner by orders. The wraith can't even leave the object unless the owner allows it to do so.
Wraiths can live indefinitely for as long as their soul object exists, and they can even change said object if it begins to deteriorate. They can never possess more than one soul object. Additionally, whoever commands the soul object commands the wraith.