immortality, though it carries with it the stench of Chaos, for demons are said to have first taught men the methods of creating -- and joining -- the ranks of the undead.
Because of their demonic pedigree, all human religions consider the undead their special foes, which is why traveling clerics are taught magical rituals for banishing or "turning" these foul creatures. Experienced clerics, infused with the power of Law, can even learn to destroy undead utterly, reducing their bodily vessels to dust and sending the human souls encased within to final judgment before the gods. This is why most undead hide themselves far from civilization, in areas where Law holds little sway.
While many intelligent undead choose their state by recourse to various evil rituals, not all do. Some become so as a result of transferring their Chaotic taint to their victims, who rise as undead of the same sort as their destroyer. These unfortunate individuals find their minds as well as their bodies twisted by the curse of undeath, becoming little different than those who sought to become undead in order to escape death. Precisely why this happens -- and why the gods allow it -- is a mystery that has baffled theologians and scholars alike since time immemorial.