These beings are usually pattern corrupted souls. Corrupted by their own inner corruption or by the hand of necromancers. Though they have no physical shell, they can sometimes use magical effects to affect those around them.
The strongest of these undead can sometimes transform their incorporeal shell, being able to inflict pattern corrupting damage to those around them. If they do this then they can then usually be disrupted (though often only temporarily) by enchanted or artefact weapons, or by High magic.