Overlords appear in Arthur C. Clarke's novel Childhood's End. They are benevolent beings that suddenly appear over every city on Earth and give out few demands: the end of war, unification of Earth, and elimination of poverty. Unlike most aliens in fiction, the Overlords do not show themselves at first and let humanity wait fifty years before seeing an Overlord.
The Overlords appear like the demons of old. They are red-skinned, winged, have stubby horns and have a barbed tail. They range from seven to ten feet tall. The presence of wings assume that they come from a place that has a low gravity field. They could breathe Earth air comfortably, yet they carry small gas cylinders occasionally. This suggest that the atmosphere of the Earth is subtly different from the one that they are used to. Their mental abilities surpass those of humanity, suggesting that their civilization has advanced much, much earlier that the human race.
Overlords might seem to be in charge of the proceedings but the fact of the matter is that they are following the orders of the creatures they call 'Overmind'. They are tasked by the Overmind to take care of mankind in order to prepare the way for their evolution and pacification. They are initially jealous of man's capability of evolution but they have come to accept it as inevitable. One of their tasks while taking care of humanity is to make sure that humanity is not capable of traversing space due to the lack of knowledge.