Gelth
The Gelth were a humanoid species who lost their physical bodies in the Time War.
Biology
The Gelth originally had biological bodies. These bodies were eventually reduced to a gaseous form during the Time War. They could shape this gas into a wraith-like form but they needed to be in a gaseous environment to survive.
The Gelth could take control of human corpses with difficulty. While inhabiting these bodies, the Gelth could control and move the dead humans as walking zombies, leaking blue glowing gas that streamed through the mouth. The Gelth would eventually be forced out of the corpse over time or if there was a lot of gas in the air around them.
Appearances
The Unquiet Dead (TV)
Scared Stiff (Prose)