Gelth

The Gelth are a race of humanoids who lost their bodies in the Time War, first shown in The Unquiet Dead.

At some point in the Time War, billions of Gelths were turned into gas. While in this gaseous state, they could shape themselves into a wraith-like form. These Gelths needed a gaseous environment to survive and would naturally move towards one. At some point, they discovered a Time Rift to an undertaker in 1869. They also discovered that they could survive in the gases produced by a decomposing body and that they could then control the corpse. Despite this, their stay in the corpse was tenuous at best and they were always pulled back to the Time Rift. They tried to get the the Doctor's and Rose's help in opening the Time Rift further.