A Toltan Moon resembled a small, natural moon, complete with soil and craters. Its surface gravity was unusually low for its size. Although it had a neutronium shell, it was mostly hollow. It was equipped with corridors full of hypersleep chambers and was capable of sending transmissions via telepathy and voice.
One Toltan Moon was placed in orbit around a seeded colony world in an uncharted sector of the Alpha or Beta Quadrant. It's mission was to monitor the colony's evolution and development. It was programmed to remain until the inhabitants had developed space travel and had accomplished a moon landing. At that point, it was to retrieve the astronauts, put them in suspended animation, and bring them back "home".
By the 14th century, the colony had developed technologically and socially to a level similar to that of Earth in the 20th century. Two astronauts flew a Toltan spacecraft to the Moon, but their society destroyed itself with nuclear weapons. The two astronauts were retrieved by an automated robot crawler and put into suspended animation. For unknown reasons, the Moon remained in orbit of the destroyed colony.
In the year 2273, the USS Enterprise detected signals from the Toltan Moon. A landing party discovered the Toltan spacecraft and were captured by the robot crawler, but they were able to escape with the two hibernating Toltans. The Toltan Moon then seized the Enterprise with an unbreakable tractor beam. The Toltan astronauts chose to return to their Moon, as cultural myths suggested that it was a vehicle of their gods. Once they were transported back, the Toltan Moon cut off the tractor beam. The automated sentinel then fulfilled its programming by breaking orbit from the colony world and heading out of the star system, returning "home". (TOS comic: "Called Home")