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 need to be in a gaseous environment to survive.

At some point, they discovered a time rift to an undertaker in 1869. They were naturally attracted to the area, since the gases found in the pipes could sustain them. They also discovered that they could survive in the gases in a decomposing human. While in the corpses, the Gelth could control and move the dead humans. The corpses slowly leaked out the blue gases, which gave them a glowing blue tint. They had trouble controlling the corpses and the humans tried to carry out their last plans. Even then, the Gelth's stay in the corpses was tenuous at best and they needed to find a better place to live in. They tried to get the the Doctor's and Rose's help in opening the time rift further.