A Treatise on the Undead

  "Undead exist simultaneously on
the Prime Material and the Negative
Material Plane. It is more correct to
say that they are linked to the Negative
Material. Undead steal some energy from
the Prime by their mere existence. They
siphon some of this energy to power their
existence. More powerful undead siphon
more power. This is called necromancy,
or entropic animacy. Other forms of
Undead are linked to the Positive
Material Plane, such as mummies.
Undead linked to the Positive Material
Plane are associated with the control
of living tissue."