When a shadow is prevented from leaving the material plane, it becomes undead. The shadow is the strand of a death bound for Malstaris, where it would cycle through Nydus and be done. Held back, it stays knotted to the body it should have left, and the body moves again. What rises carries no spirit. Without spirit there is no will, and a thing with no will is easily owned, which is why most undead do only what the working that made them commands.
This makes necromancy less the art of raising the dead than the art of keeping them. Animation is the easy part. The labor is forbidding a strand its road, and the road is forbidden in Deoric, the command tongue that pays for everything it does in life. A large host of the dead is therefore a standing debt against the living: someone bled to raise it, and someone keeps bleeding to hold it. Chaal Nazzerox runs on exactly this arithmetic.
The strands can also catch without any caster. Where many die at once, or where the First Dark that drains shadows into Malstaris runs slow, the dead pool and stand on their own. The River of Wights is one such place.
What is held, and what rises
The kind of undead follows from which strand is caught.
A held shadow animates the corpse. These are the common dead, the corporeal mass of any necromancer's army: walking carrion, skeletons strung on a single bound shadow, and the stronger wights, which keep a corpse's cunning and spite without keeping its mind. They are slow on their feet and tireless, and useless when no one is left to give them orders.
A held soul instead of a shadow makes a specter: bodiless, mindless in the same fashion, and bound as readily as any corpse. The same held soul, clamped into a fabricated suit of armor rather than left bodiless, makes a souljack.
A creature that keeps all three of its strands by will alone, declining its own death to settle something it cannot leave undone, is a revenant. Revenants have what the mindless dead lack, because they kept their spirit, and they pay for it: watching everything they loved grow old while a borrowed body rots around their resolve.
A lich keeps its spirit by breaking it. It hides a fragment in an object and wears a soul and shadow stolen from others, spending those stolen strands and replacing them at each death. Liches give the orders the mindless dead obey. Xynoth Azkonor, who rules Chaal Nazzerox from behind its rivers, is one.
Ghouls are made rather than raised, fashioned whole from a corpse by Deoric ritual and spiritless from the first breath they never take. They were built to serve, and now they serve a hunger more than any master, eating flesh and souls against an emptiness that never fills.
Not everything that looks dead is. Leather skeletons shamble like the walking dead and wear the same dried skin over bone, but they are living people hollowed out by leather-worms, a contagion in a corpse's shape rather than a death held open.