The Protos were the slaves of the Zeld. They were constructed of pure energy and appeared as misty, grey shapes. In the 1970s, a Zeld scout ship used them to drain captured humans of their psychic energy. The Third Doctor, who had been captured, witnessed one of these procedures and narrowly avoided being drained himself. The Protos were presumably destroyed when the Doctor advised UNIT to attack the Zeld base. (PROSE: The Ghouls of Grestonspey)

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