A shade is a shadow that outlived its body. When the malstaric thread of a dying creature breaks, the shadow it carried sinks to Malstaris and stays there, on the underside of the world where the shadow-dead gather. What remains is a husk of borrowed dark, the same shadow that was drawn out of Eindumor when the creature was conceived and threaded up to clothe its spirit. It is not the person who died. The person was the spirit, and the spirit has already faded toward Celestia. The shadow was only ever on loan, a scrap of Nydus's substance worn for one lifetime, now returned to the place all shadows return to.
Here is the part that misleads people. A shade keeps no spirit, so it keeps no will, no errand, no grudge it might carry back toward the living. A shadow is not empty the way a freed soul is empty, though. A shadow holds secrets. Every lie the dead person told, every thing they buried, every depth they kept turned away from the light, all of it stays folded into the husk after the person is gone. The shade does nothing with what it holds. It cannot. But it holds, and that holding is the whole reason Malstaris is the sort of place that has to be governed.
The Malstaric kingdoms of the dead
Every shade ends in the same place, because every shadow does. Malstaris lies beneath the Wastes, the underside of the material slab, a country of gray caverns and grayer plains under Nydus, the Black Sun. The shadow-dead come there in their millions and move through it in crowds that never thin. There is no hunger in Malstaris and no thirst, no weather and no sleep, and the passion is drawn out of everything that enters until a shade wants nothing and feels less. These are the Malstaric kingdoms of the dead. They are the shadow-counterpart of the Astral kingdoms where the freed souls drift, the dead of the two suns kept on the two undersides of the world.
A holding-cavern below the gray plains, lit by nothing, the walls sweating a cold that has no source. The shades stand in their thousands, flat against the rock and flatter against each other, faces angled at no particular point. They do not push and they do not speak. Now and then one drifts a few feet and settles again, the way silt shifts in still water. Every one of them is carrying something it will never say.
Why they are ruled
The word kingdoms means more here than it does above. In the Astral, the soul-dead answer to no one, because a will-less soul cannot be commanded and a crowd that only drifts is not a realm. Malstaris is the other case. A shade holds secrets, and a thing that holds secrets can be hidden, hoarded, traded, set to a use it has no say in, and lost track of if no one keeps the count. So there is something here to keep, and there are powers that keep it.
The Lord of the Dead holds a sprawling palace at the center of the order, and the shadows are looked after within it. The Parliament of Shadows governs the largest region of the plane, its seats filled by the lords of the smaller regions it gathers under itself. And the shadows are made to arrive and made to stay. Anubis sees to the arriving: he ensures the shadow of each death reaches Malstaris, weighs the soul it came with, and sorts where it belongs, the Gardens of Glyss inside the palace or the Black Wastes that are everything beyond. The staying is left to the umbral agents, shadow that was never alive and never anyone's, who herd the shade-crowds for the Lord of the Dead and the Parliament and turn back any husk that drifts out of the region set for it. None of this is mercy and none of it is punishment. It is administration, the keeping of an enormous archive that happens to be made of the dead.
Outside the Black Citadel, on the cliffs of the Breakneck Canyons, the line of the dead runs back for miles. The shades wait their turn without restlessness, because restlessness is a thing the living do. They came here to be admitted, one at a time, to whatever the vampire lord of the tower wants with them. The front of the line moves. The back of the line cannot see the tower at all. None of them will leave the way they came.
The long holding
A shade is no more permanent than a ghost. How long it lasts turns on how much it still hides. A shadow heavy with unspilled secrets can persist for centuries, sometimes for thousands of years, because the dark in it stays thick and defined. A shadow worn thin, its secrets all spent or spilled or simply emptied by time, drifts at last past the failing edges of Malstaris into Eindumor, the raw presence of Nydus that lies beneath the plane. There it loses the last of its shape and dissolves into the undifferentiated dark. In time that dark is drawn forth again as a new shadow for a new life, carrying nothing it carried before. Shadows cycle, the way souls do. A shade is a long holding and not a final one.
A few never dissolve on schedule, or at all. The oldest and most secret-laden hang on far past their reckoning, and the very first shadow ever to sink to Malstaris, the shadow of Lyzaria, never thinned at all. It rules a region of the plane that no other power dares enter. Whatever it is now, it is no longer a shade in any ordinary sense. Shades that drift into its region do not drift back out.