The Gelth are the main antagonists in the Doctor Who episode; The Unquiet Dead.
The Gelth had humanoid ghost-like bodies made of blue gas and they make ghostly screams. When revealing their true colors, They turn into orange color with red eyes and speaks in demonic voices.
They lost their bodies in the Time War against the Daleks and tried to kill the Human race to use their bodies instead. A first Gelth inhabit and take over the corpse of Mrs. Peace and kills Mr. Redpath (who is the grandson of Mr. Peace) by strangling him and then snapped his neck, Gabriel Sneel try to keep Mrs. Peace in her coffin, but to no avail and went waking into the street with Gelth possessing her lifeless body.
The Gelth inside Mr. Peace's body was next seen at Charles Dickens' performance. Charles Dickens saw the Gelth and the Gelth lead Mrs. Peace's body and flying around and then went into a gas lamp.
Two Gelth inhabit the corpses of Mrs. Peace and Mr. Redpath and attempt to kill Rose Tyler, but The Doctor came in time to save Rose in the nick of time, The two Gelth tell The Doctor about to open the rift and tell him they were dying, The two Gelths then lead the human corpses of Mrs. Peace and Mr. Redpaths and went into gas lamps.
Gwyneth open the rift and release the rest of Gelth in the room, However there were a few billions much more than Gelth originally implied, That was the Gelth reveal their true colors; it turns out the human corpses was not enough for them and their plan was to kill the entire human race to make corpses in order for their kind to survive and take Earth by force. The Gelth have the Doctor and Rose corned and trapped in the dungeon.
Another Gelth went after Charles Dickens, but the Gelth went into street lamp then Charles Dickens realize something about the Gelths with gas, Charles Dickens went filling the house with gas and suggesting The Doctor to do the same thing too.
The Doctor realize if the room is full with enough gas, The Gelth will forced out of the human corpses like poison from a wound. The Doctor then release all of the gas in the room and Gelth were forced out of the human corpses by the gas and flying around in the air unable to break free from the gas.
The Doctor tell Gwyneth to sent Gelth back because them were not angels and they lie, Gwyneth realise the truth, but she can't sent Gelth back due to them being to strong, but she is strong enough to hold the Gelth in the position she is standing in, Gwyneth tell the Doctor and the others to leave the house so she can light a match in the room full of gas to cause a explosion to destroy the remaining Gelth and results in killing herself.
They were stopped when Gwyneth sacrificed herself to close the Rift which the Gelth had used to come to Earth by lightning a match with the house full of gas causing a explosion destroying the Gelth and was killed in the process.
Powers and Abilities
The Gelth have the ability to inhabit and control human corpses.