Nimon

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

Appearance
The Nimon were large black-skinned humanoids resembling the mythical minotaurs. Their horns can shoot out blasts of energy. Nimon drained energy direct from their victims, leaving them as husks. The Nimon have mighty, rumbling echoing voices.

Life cycle
The Nimon could use an artificial black hole as a gateways from planet to planet. This technology was effective but not energy efficient. Due to their high energy needs, the Nimon undertook migrations they called the Great Journey of Life. Usually, one Nimon was sent to a planet, representing themselves as a god with advanced technology where it would construct a maze-like Power Complex, a building-sized labyrinth of circuitry designed to generate a new black hole which it then used to transport more Nimon to the planet. The Nimon would then drain that world dry and move on.

History
When the Doctor and Romana stopped the Nimon they had used exhausted Crinoth and had moved on to Skonnos, home to the faded Skonnos Empire. The Nimon used the Skonnon Soldeed as their priest and demanded tribute of hymetusite crystals and demanded tribute from the planet Aneth. (DW: The Horns of Nimon)

The Doctor later encountered the Nimon who were in contact with Sebastian Grayle influencing him to aid their invasion of Earth. (BFA: Seasons of Fear)

Behind the Scenes
Anthony Read, the creator of the Nimons drew heavily from the ancient Greek myth of the minotaurs when creating the Nimons.

When making The Horns of Nimon production team had conceived the Nimon's 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.