Vrall

The Vrall were a race of ruthless monsters who were almost unleashed on Earth in the nineteenth century.

Biology
As a race, the Vrall were utterly ruthless, highly effective at disguise, and nearly unstoppable. They appeared to be just a very dangerous alien animal used for sport on an abandoned hunting ground on the Moon, but were actually sentient and incredibly cruel. They were shaped like men, but were about half the height of humans and could bend and flex themselves to only a hand’s thickness. They could change colour to blend into their surroundings. They could also link together to form an even more powerful entity, they didn't bleed, and any small cuts or punctures quickly healed over. The only way to stop one without advanced weaponry was to either dismember one (difficult since they moved so fast) or set off an all-consuming fire (also difficult, given the need to blow up something pretty big to stop them running out of it). (PROSE: Imperial Moon)

History
The park in which the Vrall lived had been used as a hunting ground for an apparently long-gone empire, populated by deadly beasts and controlled by something called the warden. The atmosphere was controlled by a force field around a large crater, keeping air in but letting solid objects leave. The Vrall used a species called the Phiadorans, exiled to the Moon, as hosts. They soon became hosts to the Vrall and remained that way for some time until the year 1878.

Around that time, the Vrall took one desperate chance to escape; they had seen Earth when the Moon was just right, but knew that humanity would take too long to develop space travel by themselves to be of any use. They created miniature RNA spores encoded with the construction information for a space engine, which they fired out of the dome around the park with a vacuum gun. Eventually, one spore struck the mind of a Professor Boyes-Dennison, who developed the engine with the aid of his twenty-two year old daughter Emily. The engines were used on three ships — the Cygnus, the Draco, and the  Lynx, which were commanded by Captains Richard Haliwell, Sinclair and Green, with Haliwell in overall charge.

After arriving on the Moon (more specifically, in the park where the Vrall were), the original expedition lost one of their men, Sub-Lieutenant Granby, and encountered the Fifth Doctor and Turlough. Posing as a scientist and his assistant, the Doctor and Turlough helped the expedition find Granby. He had, quite literally, had his brains sucked out by the Vrall, who had attacked him in order to absorb his knowledge and learn the Earth language.

While trying to get to the main citadel with the Doctor and Turlough, the British party encountered one Vrall in Phiadoran form, apparently in danger. Turlough saved her. She claimed that she took knowledge of his language from his mind to explain her having previously spoken in some kind of odd wailing noise, really just her adapting to English. Calling herself Lytalia, she took the team to the home of the Phiadorans, where they decided to help the British team stop the Warden. The warden had a large force field around its citadel that prevented creatures with a certain DNA pattern entering. Since human DNA hadn't been entered into the force field, they could get at the warden. With one Phiadoran per member of the team, they got to the warden's base. The Doctor reprogrammed two of the warden's flying sentries so he and Turlough could pilot them, allowing them to penetrate the Citadel, rescue Haliwell and Emily, and convince the Warden to deactivate the defences. As the citadel began to collapse, the British soldiers raced back to their ships, as the force field that kept the air in the crater began shutting down with the warden's death.

The Doctor, Turlough, and Lytalia became separated from the rest, and barely managed to find their way to the crashed Draco, only to discover that the helmsman, Henry Stanton, had mutinied and only a few crewmen were left. The Doctor got the ship up, but was kicked out by Stanton's crew, leaving Turlough alone on the ship, with the Vrall/Lytalia picking them all off. Kamelion found the Doctor before he could suffocate, and they were able to rescue Turlough before Stanton drained the air out of the Draco to stop the Vrall.

During his time recuperating in the TARDIS, the Doctor worked out the truth about the Vrall as his subconscious was able to focus on the anomalies that he had only briefly registered at the time, such as their use of the term "mechanical servant" to describe the Warden's robots; "mechanical servant" was a term that would have been used by Sub-Lieutenant Granby rather than Turlough, resulting in him realizing that the Phiadorians/Vrall had absorbed knowledge of English from Granby's mind rather than a telepathic download from Turlough. The Doctor and Turlough subsequently located some weapons on the Moon that could hurt the Vrall, and destroyed them all, with all records of the expedition being destroyed in the aftermath. (PROSE: Imperial Moon)