A specter is what a soul or shadow becomes when necromancy strands it in the overlay it should have crossed. How that stranding happens, the cutting of a thread mid-transit so the remnant can neither finish rising nor sink back, belongs to the entry on the specter as a process. The concern here is the thing left behind.
What is left is never the whole of a person. The spirit, the seat of will and self, rides its own strand up to Celestia at death and ends there. It does not stay. So a specter is a soul or a shadow with nothing standing behind it that wants anything. It has substance enough to act and no will of its own to spend that substance on. The thread-binder who made the cut holds the remnant afterward, and a thing with nothing left in it to refuse is the most obedient servant a necromancer can own.
Specters come in two kinds, and the kinds differ in feel, not in nature. Which one is in front of you depends only on which overlay the death was crossing when the thread was cut.
The soul-specter
A soul severed in the Ethereal overlay strands as a soul-specter. It keeps the cast of the strand it was riding. The Ethereal is the lighter of the two overlays, and soul-work done there carries the same luminous, clarifying quality a healer's mending does, so a soul-specter is a pale thing, lit faintly from within, and unnervingly calm. It does not rage. It does not mourn. It stands where it is put and does what it is told with the placid attention of someone who has forgotten there was ever anything else to do.
That stillness is the danger of it. A soul-specter set to watch a road will watch the road for a hundred years and raise no alarm it was not instructed to raise, and it will not be bargained with, because there is nothing inside it that wants to live or to be let go. The few who have tried to reason a soul-specter loose of its binding describe the experience as talking to a lamp.
The shadow-specter
A shadow severed in the Nethereal overlay strands as a shadow-specter, and it is the colder thing. The Nethereal runs dark with malstaric energy, the layer where secrets and violence leave their deepest marks, and a shadow caught crossing it comes away heavy with whatever it carried into death. A shadow-specter is cold to stand near. It holds what the dead person hid. Necromancers prize the kind taken from someone who died mid-betrayal or mid-confession, because a shadow-specter can be made to give up a secret its living owner would have died to keep, and several of them did.
Where a soul-specter is placid, a shadow-specter is attentive in a way that reads as malice and is not. It has no malice. It has only the residue of a shadow that spent its last moments fixed on a hatred or a fear, and it keeps facing that direction the way a compass keeps facing north.
I asked it three questions. The first two it answered in my mother's voice, which it had no right to, because my mother is alive. The third it would not answer, and I understood then that the binder had not given me a servant. He had given me a thing that knows when it has been told enough. — a confession recorded at the edge of Chaal Nazzerox, speaker unnamed
The craft and its master
Stranding a specter on purpose is the precise end of necromancy, and most necromancers never manage it. The bulk of the trade is coarse work, a shadow clamped back to a corpse to make a laborer or a soldier. Cutting a thread at the exact moment of transit, so the remnant strands clean instead of tearing, takes a thread-binder of rare control. It is Deoric like all such work, the titans' command tongue, and it bills the caster in his own life for every cut.
The name most attached to deliberate specter-work is Vazreth Sythrox, a thread-binder who served the lich-king Xynoth Azkonor at Chaal Nazzerox and refined the stranding into something close to a method. He is also the cautionary tale of the craft. Sent up the River of Wights to bind the masterless wight-lord Mylanor, he did not come back, and the manner of his not coming back is told on his own page.