A ghost is a soul that outlived its body. When the astral and malstaric threads of a dying creature break, its freed soul floats up to the Astral Plane and stays there, the way Life and Death describes every soul rising at death. What remains is the bare strand of Aurus-light that once helped power a living thing, now unhoused and energy without a vessel. It is not the person who died. The person was the spirit, and the spirit has already faded toward Celestia. The soul was only ever borrowed essence, the same light that will one day be drawn forth to power some other life.
This is the part most people get backward. A ghost does not haunt. It holds no spirit, so it has no will and no remembered grievance, nothing to drag it back toward the room where it died. The thing that lingers in a house with a purpose and a grudge is not a ghost at all but a revenant, a being that refused to let its spirit go. A true ghost wants nothing. It only drifts.
The astral kingdoms of the dead
Every ghost ends up in the same place, because every soul does. The Astral Plane is a sea of floating islands lit from above by Aurus, the White Sun, and the dead gather there in their millions. On the larger islands stand cities, and the soul-husks mill through them in crowds that never thin and never break up. These are the astral kingdoms of the dead. The name overstates them.
A street in one of the high astral cities. The buildings are real, raised by living hands that crossed over to build them, but no one keeps the streets and no one walks them with anywhere to go. The dead move through in slow currents, faintly luminous, faces turned nowhere in particular. A crowd that never disperses and never quite settles. None of them will remember being here.
A ghost stays in the Astral for the reasons set out in Life and Death. The planar forces of the stack push souls up and hold them; no thread binds a ghost down to a body, so nothing draws it back; and even if a road home opened, a soul has no will with which to choose to take it.
Why no one rules them
The word "kingdoms" invites a comparison the Astral does not earn. Down in Malstaris, the shadow-dead are governed. There is a lord in a palace, and a standing order that keeps the shadows where they belong. The Astral kingdoms have nothing of the kind, and not because no ruler has claimed them. There is simply nothing to rule. Souls give no obedience and answer no command. They need neither feeding nor keeping. The cities take shape and shift on their own, currents in a crowd rather than realms with a throne. No one rules the dead of the Astral because the dead of the Astral cannot be ruled.
The long pause
A ghost is not permanent. A soul persists in the Astral for a stretch that can run to centuries or longer, and then it drifts down through Astraeva, the bare presence of Aurus beneath the Astral Sea, where it dissolves back into raw light. In time that light is drawn forth again as a new soul for a new life. Souls cycle. Set against that, a ghost is a long pause in the turning and not a stop.
What it pauses in is older than any of them. The Astral Sea is made of the essence of Lyzaria, the first titan to die, and every ghost adrift in it moves through traces of her memory without the will to notice them.