The Gelth were a gaseous life form first seen in The Unquiet Dead (Series 1 : Episode 3) in Victorian Era Cardiff. The Gelth used to have bodies, but these were destroyed and they were turned into gaseous life forms during the Time War. The Gelth needed a gaseous form to survive, and often inhabited the bodies of recently deceased Humans, who would then appear to have returned from the dead. The Gelth contacted the Ninth Doctor through Gwyneth, who had psychic abilities having grown up near the Rift in Time and Space in Cardiff. The Gelth asked the Doctor for help, and convinced him that they were facing extinction, and wanted to use the bodies of dead Humans to survive. Though this idea was not well received by his companions, the Doctor agreed, and said he would take the Gelth to a new home where they could specially-create bodies to use. However, when the Rift was opened, billions of Gelth tried to get through, with the intention of taking the planet Earth by force. They were stopped by Gwyneth, who closed the Rift to them and destroyed those remaining with fire (by blowing up the house).

The Gelth gas appearing in Humanoid form

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