Nimon

The Nimons were a species of parasites on a galactic scale, using up civilizations and moving on.

Appearance
The Nimons were large black-skinned humanoids with masks resembling the mythical minotaurs. Their horns could shoot out blasts of energy. Nimons drained energy directly from their victims, leaving them as husks. The Nimons had mighty, rumbling echoing voices.

Life cycle
The Nimons would feed on the population of an entire world, usually consuming it to extinction within a few years. As their food source got scarce, they would look for another planet ripe for conquest and draining.

Usually, one Nimon was sent to a planet, representing itself as a god with advanced technology. It would construct a maze-like Power Complex, a building-sized labyrinth of circuitry designed to generate a pocket black hole back to the Nimons' current planet. The Nimons would then migrate en masse to the new planet using capsules transmatted through the black hole. The Nimons would then drain that world dry and look for the next, continuing the cycle. They referred to this as the Great Journey of Life. (DW: The Horns of Nimon)

Technology
Nimons were able to transport capsules between worlds without the use of black holes, but the prospect used a considerably larger amount of energy, and was only possible when those planets were in a particularly beneficial galactic alignment. The process was made much easier if they could construct any sort of "ground station" on the invading world. Once a Power Complex was built to generate a black hole, alignment of the planets was no longer necessary.

Among the tools the Nimons had were psionic mind beams and gravitational lensing. With a black hole established, they could phase energy and matter through its event horizon.

Besides being able to draw energy from the binding forces of organic matter (people), they could also transmit energy in the opposite direction. The process was capable of providing a person near-immortality, and is theorized could make a person fully immortal and invulnerable if given a full charge. (BFA: Seasons of Fear)

History
The Nimons were one of many races that participated in the Millennium War against the Mad Mind of Bophemeral. They collaborated with the Osirians in dropping quantum collapsers on the insane computer intelligence. (PDA: The Quantum Archangel)

In Earth's ancient times, Earth had seen the appearance of a Nimon scout. Before the scout could set himself up as a god, he was attacked by a group of heroes, led by the warrior Mithras. The heroes managed to take the Nimon's own sword (specially tempered by travel through a black hole) and kill the beast with it. The tale of Mithras slaying the Bull became legend with the advent of Mithraism. (BFA: Seasons of Fear)

Around 300 AD, the sword and a communicator were holy relics of the Mithraic religion, being used in ceremonies in Roman Britannia. The Nimons managed to contact Sebastian Grayle, a legionnaire, and offered him immortality if he would set up the ground station and sacrifice needed to establish a link between the Ordinand System and Earth. The Eighth Doctor arrived shortly before the sacrifice and thwarted the plan, but not before giving Grayle near-immortality. Over the next 1500 years the stars aligned properly for two more attempts, but the Doctor was on hand at each time to prevent Grayle and the Nimons from succeeding. Although they almost succeeded in conquering Earth, the Doctor was able to avert the invasion by taking the future Grayle back in time to confront his past self at a point when the energies that gave him his immortality had been depleted, the past Grayle killing his future self because he was outraged at his future self's behaviour. (BFA: Seasons of Fear)

Sometime in the far future, the Nimons had finished with their latest planet Crinoth, and were looking to migrate to Skonnos, home of the faded Skonnan Empire. The Nimon scout set himself up as the local god, promising the Skonnan leader Soldeed the power he would need to make Skonnos a great empire again. Soldeed demanded a tribute of hymetusite crystals from neighbouring planet Aneth for the power source. The Fourth Doctor and Romana happened upon the planet, and stopped the migration with the help of human sacrifices also sent from Aneth, leaving the Nimon trapped on Crinoth. (DW: The Horns of Nimon)

Behind the scenes

 * Anthony Read, the creator of the Nimons drew heavily from the ancient Greek myth of the Minotaurs when creating the Nimons. (Minotaur had appeared earlier in Doctor Who in The Mind Robber and The Time Monster.)
 * When making The Horns of Nimon production team had conceived the Nimons' large "heads" as masks that would open up, revealing their desiccated actual faces beneath. However, the budget could not stretch that far and so they never attempted to show that the Nimons had a hidden true form.
 * It is a long-held fan myth that the plural of "Nimon" is "Nimon". This mistake is understandable, because initially most of the characters in Horns don't know there's more than one Nimon present. Thus, they refer to "the Nimon". However, as the story progresses, the script makes it perfectly clear that the plural of "Nimon" is "Nimons".