Wraiths, also known as Boggarts, are a subclass of Ghosts. They have been tainted by negativity and, because they've taken in aspects of others and different qualities, have gone 'off the rails' so to speak. They are quite hostile and dangerous compared to what they once were, driven by echoes different than their own.
Wraiths are so far removed from ghosts that they can cause interference in the physical world rather than simply being an impression. They don't have a traditional weakness to salt because of this, but they are often violent and tend to act within a criteria even at the urgings of a practitioner.
Wraiths tend to either burn throught the power they have, leading to their own self-destruction, or they can be subdued by resolving all the agression they have that serves as their echo, much like any other ghost. This usually involves getting their revenge.