Gelth

The Gelth were once an unknown humanoid race that has long since been dead due to the Time War and what was left of them was turned into gaseous creatures.

Biology
The Gelth are very ghost-like in appearance and abilities with their transparency and bioluminecence, their ability to float, gaseous state, etc.

Abilities
With how the Gelth exist in a gaseous state and how they can't exist without the presence of gas, The Gelth is unable to be harmed by any physical means and can inhabit anything with gas, including human corpses. When a Gelth possesses a human corpse, they are able to act and move like a reanimated human and often use the possession of dead bodies to kill humans by snapping the neck.

History
The Gelth were a race of a currently unknown humanoid alien species before they were eradicated during the Last Great Time War. Under unknown circumstances, what was left of the Gelth were turned into a gaseous state. They discovered a rift that brought a few of them to Earth in Cardiff during the Christmas of 1869. Since they couldn't live with out the presence of gas the Gelth that traveled to Earth hid within the pipes fueling gas lamps in the walls of houses. Later, due to the gas produced by decomposing corpses, the Gelth discovered that they can possess the bodies of dead humans. This gave the Gelth the reputation as ghosts, as well as their gaseous state. The Gelth decided to use their inhabiting of corpses as a method to have the Gelth continue to survive and reign as a species. After briefly gaining the assistance of The Doctor to find the psychic "Gwyneth" to open the rift wide enough to let the Gelth through to Earth and possess the dead. The Doctor wasn't aware of how many Gelth were waiting on the other side, so when billions of Gelth went through the rift, it was revealed that the Gelth planned to use the dead bodies to kill enough humans on Earth for the Gelth to inhabit. With the help of Charles Dickens, who turned off all the gas lamps nearby which drew the Gelth out into the open, Gwyneth lit a match to burn every Gelth on both sides of the rift in a massive explosion with The Doctor, his companion Rose Tyler, and Dickens escaped with their lives.

Appearence

 * (revived) Doctor Who Season 1 Episode 3 "The Unquiet Dead" (2005)