Undead are horrible monsters that originate from the Plane of Soul - the Dead Lands specifically - yet are able to reproduce and replicate with living beings, including and especially Sentients, typically through infection or some unnatural conversion process. Undead creatures are called such because of the state of their existence: they are neither living nor dead, yet they function, have goals, follow orders, and act on their instincts. Undead are feared by many, and not without good reason; the appearance of Undead outside of the Plane of Soul is almost never a good thing, and bloodshed is almost certain to follow.
Throughout the Dead Lands, Undead prowl, looking for Specters to consume and destroy for good. Most lower-Tier (and, not coincidentally, lower-INT) Undead, such as Zombies and Ghouls, simply roam and aimlessly look for Specters to consume, and on their own are no major threat, though packs of them can quickly become a major problem. Higher-Tier/higher-INT Undead, like Vampyre Lords, plot, plan, and scheme their way to consuming Specters, and in so doing are usually much more successful and much more feared.