A wight is a corpse that walks with more of its old self than it has any right to. A skeleton is a single shadow strung on dead bone, doing only what it was told. A wight's held shadow caught the spite and the cunning of the person it belonged to. Not the mind. Not the will. The self faded to Celestia at death, the way every self does, and it is gone. What stayed behind in the shadow is the residue of a temperament: the grudges, the low cleverness, the habit of giving orders that the corpse still answers to. A wight remembers how to be dangerous without remembering who it was.
The wider account of how the dead are held belongs to the Undead. What sets a wight apart is the kind of shadow, not the working that raises it. A shadow sinks toward Malstaris carrying whatever it went into death holding, and the ones thick with secret and hatred persist the longest. Clamp such a shadow back to its body and the thing that stands up rises sharper than the common dead. The same Deoric thread-work that makes a mindless skeleton makes a wight; the difference is whose shadow you used.
Most wights are made on purpose. A thread-binder reaches through the Nethereal overlay in Deoric, the titans' command tongue that takes its price in the caster's own life, and forbids a spiteful shadow the road it was already taking. The result is the one part of a necromancer's host that can be trusted with an order more complicated than walk forward. Wights pass commands the mindless dead cannot parse and enforce them on the bodies that can. They are the field officers of any large host of the dead. Chaal Nazzerox, the necromancer-state that runs its whole economy on held shadows, fields them by the company.
Wights also rise where no one calls them. In the northwestern Crimson Coast the First Dark, the current that should drain spent shadows down into Malstaris, runs slow, and the violently dead pool there instead of leaving. The River of Wights is the pool that earned a name. The shadows that stand up out of its upper reaches owe nothing to any caster, which makes them the worse kind of host: you cannot end them by killing the necromancer who raised them, because none did.
You do not pull a body from the river after dark. Not for burial, not for the coin in its pockets, not if it wears your brother's face and calls you by your name. Your brother's shadow caught wrong. It has only learned to use his voice. — a rule taught among the lower-river fishers
The river country has produced the rare wight that gives orders instead of only taking them. Mylanor, a Winter Elf lord killed in the early years of the succession war, rose from the upper water with cunning enough left in his shadow to command the dead around him. Both halves of the broken kingdom have tried to put him to use. Neither has managed it.