Phantoms are similar to, but distinct from ghosts. The difference is that they are able to incarnate their bodies at will. Phantoms are only able to be killed by decapitation or heat and only when they are incarnated, they are able to return to their ghost-like form with equal ease, making them difficult to kill.
They also have many abilities such as telekinesis when in phantom form. They are a very powerful and small part of Lore, meaning their weaknesses are not well known.
It is implied by Nix, that a ghost who remains in their spirit anchor long enough may transform into a Phantom after a sufficiently long period (around a millienium).