Of the Wamphyr, little is known.
Urtha is the only one encountered, and from her comments a Wamphyr appears to be a Vampire that was once slain, only to rise again. It was "mystic fire" which caused Urtha to rise once more from the earth. This mystic fire was also said to be animating the Haggwort that she controlled, but she appeared to do this via an iron wristband, not necessarily by virtue of being a Wamphyr or any other magical link.
Special means are required to put an end to a Wamphyr. Urtha, as a vampire, was slain by a stake, but as Wamphyr possessed no heart and would reappear even after suffering decapitation.