It's beginning to look a lot like Christmas - WNAR#79

Monster – The Gelth
Episode – The Unquiet Dead
Planet of Origin – Unknown
Technology -  as gaseous creatures they poses no technology

Though the Gelth originally had biological bodies, they were eventually reduced to a gaseous form during the Time War, after their bodies were destroyed during the conflict. They could shape this gas into a wraith-like form, but they needed to be in a gaseous environment to survive. Once there, the Gelth emitted ghostly screams as they floated around the location at which they congregated.

The Gelth could take control of human corpses, though only with difficulty. While inhabiting the corpses, the Gelth could control and move the dead humans as walking zombies, leaking glowing blue gas that streamed through the mouth. While controlling a human, they tended to continue unfinished business. 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.