The Draugr are armed undead, sometimes produced by agents of the Underworld, other times simply the products of Necromancy.
A Draugr is essentially a reanimated corpse inside a suit of armor, possessed by the spirit of the warrior whose body it is. When this happens to a person wearing Dolahan armor, they are called Thusen Draugr. Unlike necromanically-produced Draugr, Thusen Draugr come back on their own.
Honoreck, the lich-king who ruled the Grey Temple necromancers during the Necromanic Wars, was a Thusen Draugr, widely considered to be a major part of his success and great power.