Death has many forms.   Some are ethereal, bridging the gap between the underworld and the physical world.   Others are solid flesh and bone which are driven to persist after death by some twisted magic or perversion of soul matter.   Some Undead are benevolent souls with a mission to protect or warn, others are twisted by strong emotions such as anger, guilt, remorse, or love.  Then there are those who willingly shed their mortal coil to become something twisted and dark - set on the eradication of all life on earth.