The Undead are a species of Others that are made from the dead and dying. Formed through impressions, grudges, or by disturbing the balance between life and death, these Others were often once Humans or Practitioners.
The art of using the undead is called Necromancy.
The Undead don't change much from their original appearances while alive, besides whatever toll was taken on their body. So far there have been:
Any usefulness that stems from an undead is from the natural abilities they had in life or as basic muscle.