Monster Files: Gelth

The Gelth were intelligent gaseous lifeforms, blue and spectral in nature, who claimed to have lost their corporeal forms as a consequence of the Time War. They arrived on Earth via the spacetime rift at an undertaker's house in 1869 Cardiff and proceeded to take possession of recently deceased corpses. Their forms could not be maintained for long in Earth's atmosphere and they required a gaseous medium to sustain them — gas from decomposing bodies or coal gas in the gas pipes common to Victorian era households.