Within the Realms Supernal, all things are possible and all boundaries are shattered, even the boundary between life and death. Those versed with the Death Arcanum insist that when the soul dies, it loses that vibrant spark that makes Awakened magic possible, unless a mage finds a way to circumvent that boundary as well to return as spectral creatures from the Twilight.
Ghosts are driven by passions that sustain them beyond death. Some mages believe that a passion tied to a handful of spells, or even an Arcanum of magical influence, is strong enough to defy that very limitation. A living mage possess the strength of will to pursue nearly any form of magic. A ghost capable of performing magic is limited to a handful of rotes and spells, clinging to them like the earthly place it haunts. Like every ghost, a ghost mage is determined to preserve his anchors and retains very little free will. If a ghost mage triggers Paradox, the anchor will be afflicted by it. This leads the ghost mage into destroying what he once loved, unless someone stops him.
Mages not only have the insight to detect ghostly mages, but the countermagic to oppose them. The existence of such horrors is merely a sign that true magic, like true evil, is not limited by the reality we know.