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. They also make ghostly screams.
They lost their bodies in the Time War againt the Daleks and tried to kill the Human race to use their bodies instead. A first Gelth inhabit and take over the dead body of Mr. Peace and kills Mr. Redpath (who is the grandson of Mr. Peace) by strangling him and then snapped his neck, Gabriel Sneel try to put the leed on her to keep her in the conflict, but she got free and went waking into the street.
The Gelth inside Mr. Peace's body was next seen at Charles Dickens' peformance about ''A , 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 dead bodies of Mrs. Peace and Mr. Redpath and try to kill Rose Tyler, but The Doctor came in time to save Rose, 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, All the Gelth inhabit all dead bodies in the room and was going kill Rose Tyler and The Doctor.
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 went to tell The Doctor.
The Doctor realize if there lot of gas around the Gelth, The Gelth will forced out of the human corpses. 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 able to break free from the gas.
They were stopped when Gwyneth sacrificed herself to close the Rift which the Gelth had used to come to Earth.
Powers and Abilities
The Gelth have the ability to inhabit and control human corpses.