The Ethereals were a humanoid race native to an Earth-like planet. Their capital city was Droog. The Ethereals built the Baggolts to serve them, but the Baggolts rebelled and turned their former masters into living clouds in a failed attempt to vaporise them.
Five eketrons later, the First Doctor visited their world, where the Ethereals captured him and tried to turn him into a mould for new humanoid bodies, a process which would have killed him. Instead, the Doctor restored their original bodies, then helped their leader Mitzog put an end to the Baggolt rebellion. (PROSE: The Cloud Exiles)