Nimon

The Nimons were a parasitic species that operated on a galactic scale, using up civilisations and moving on.

Biology
The Nimons were large, black-skinned humanoids with bull-like heads, similar to the minotaurs of Earth's myths. Their horns could shoot out blasts of energy. Nimons drained energy directly from their victims, leaving them as husks. They had mighty, rumbling, echoing voices. (TV: The Horns of Nimon) They were related to another minotaur-like species. (TV: The God Complex)

Culture
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. (TV: The Horns of Nimon) The Nimons were very keen to conduct the Great Journey of Life to Earth, due to its prime position in the galaxy. From there, various alignments could offer millions of worlds for the Nimon migration to attack. (AUDIO: Seasons of Fear)

Technology
Nimons were able to transport capsules, a similar idea to transmat capsules, between worlds with the use of black holes linked by a space-time tunnel, 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.

The Nimons would direct a beam of energy to create the black holes which were a gateway into hyperspace, where it would link with the other black hole, creating a beam which provided the motive power for the space capsules. (TV: The Horns of Nimon)

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. (AUDIO: Seasons of Fear) They also used gravity devices. (PROSE: GodEngine)

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

Spreading throughout the universe
The Nimon homeworld was a planet inhabited by Nimons. Exile from it was considered a cause for shame. (COMIC: Space in Dimension Relative and Time)

The Nimons were one of the many races that participated in the Millennium War against the Mad Mind of Bophemeral. They collaborated with the Osirans in dropping quantum collapsers on the Mad Mind. (PROSE: 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 a new religion. By 305 AD, the sword and a communicator were holy relics of the Mithraic religion, being used in ceremonies in Roman Britannia. (AUDIO: Seasons of Fear)

Sometime in the 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 his companion Romana II happened upon the planet, and stopped the migration with the help of human sacrifices also sent from Aneth, leaving the Nimons trapped on Crinoth as it exploded. (TV: The Horns of Nimon)

The Nimon invaded a planet, and took its princess, Jahanna, captive. They sent a wormhole device to another world in preparation for the next invasion. Unbeknownst to them, a native of the other world, Jak had snuck through the wormhole. Jak saved the princess by knocking one of the Nimons unconscious. He then escaped with Jahanna back to his world and destroyed the Nimon's wormhole device. (PROSE: Jak and the Wormhole)

Trying to become the Lords of Time
When the Eighth Doctor saved Charlotte Pollard from dying aboard the R101, the aftereffects that rippled throughout the Web of Time came to the attention of the Nimons. Intending to use these fluctuations to spread throughout the universe, planning to conquer Earth first due to its prime location, and then supplant the Time Lords from their position of power over time, the Nimons contacted the legionnaire Sebastian Grayle in 305 AD 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. Having encountered the Nimon-controlled future, the Eighth Doctor and Charlotte arrived shortly before the sacrifice and thwarted the plan, but not before it gave 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. When the Nimons finally broke through, the Doctor realised their technology was more advanced than his first meeting with the beings and that the aliens themselves appeared to be larger. Although they almost succeeded in conquering Earth, the Doctor was able to avert the invasion by taking the future Grayle, who had lost his immortality, back in time to confront his past self in 305 AD. Disgusted at the sight of what he would become in the future, the past Grayle killed his future self, vowing to never become him and averting the chain of events that led to him becoming the Nimons' servant.

Meanwhile, Grayle's fellow Roman soldiers, believing the Nimons to be the demons the warrior Mithras had slain, cut down the Nimons that travelled back with their servant. With the Nimon-controlled future averted, the Doctor later told the Matrix Rassilon of these events. (AUDIO: Seasons of Fear)

Alternate timeline
In an alternate timeline, the Nimons succeeded in their invasion of Earth, using that prime location to spread and eventually replace the Time Lords and become total masters over space and time. In this timeline they even possessed to power to create other alternate timelines, illusions to mask the reality that they controlled. The Eighth Doctor's interference with Sebastian Grayle prevented this timeline from occurring. (AUDIO: Seasons of Fear)

In another alternate timeline, a lone exiled Nimon drained the planet Datastore 8 of all its resources, knowledge and energy to create a black hole with the intention of destroying the Nimon homeworld. It then called the Doctor's TARDIS to use its engines to activate the bomb. The Eleventh Doctor undid this by going back in time and preventing the Nimons from subjugating Datastore 8. (COMIC: Space in Dimension Relative and Time)

Behind the scenes

 * Anthony Read, the writer of The Horns of Nimon, drew heavily from the ancient Greek myth of the Minotaur when creating the Nimons. (The Minotaur had appeared earlier in Doctor Who in The Mind Robber and The Time Monster and later The God Complex.) This became the reason they were named to be cousins in the television story The God Complex.
 * When making The Horns of Nimon, the 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 believe there is only one Nimon. Thus, they refer to "the Nimon". However, as the story progresses, the script makes it perfectly clear that the plural of "Nimon" is "Nimons".