Vrall

The Vrall were a race of ruthless monsters who came close to being 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 that was used for sport on an abandoned hunting ground on the Moon, but they were actually sentient, although incredibly cruel. They were shaped like men, but were about half that height and could bend and flex themselves to only a hand’s thickness and change colour to blend into their surroundings. They could also link together to form an even more powerful entity, didn’t bleed, and any small cuts or punctures quickly healed over. The only way to stop one without advanced weaponry was to either completely dismember one (difficult since they moved so fast) or set off an all-consuming fire (also difficult, given that you’d to blow up something pretty big to stop them running out of it). (PDA: Imperial Moon)

History
The park the Vrall lived in 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 as hosts a species called the Phiadorans, who were exiled to the Moon. They were quickly taken over as hosts for the Vrall and remained that way for some time until the year 1878.

At around that point, the Vrall took one desperate chance for 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. With little chance of success, they created miniature RNA spores encoded with the construction information to a space engine, which they fired out of the dome around the park via a vacuum gun. Some of the spores didn’t do anything, but one spore struck the mind of a Professor Boyes-Dennison, who managed to develop 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 Haliwell, Sinclair and Green, with Haliwell in charge overall.

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 - who had, quite literally, had his brains sucked out. This had been done by the Vrall, who had attacked him in order to absorb his knowledge and learn the Earth language, although it would take some time for them all to understand it.

While trying to get to the main citadel accompanied by the Doctor and Turlough, the British party encountered one Vrall in Phiadoran form, apparently in danger. Turlough saved her, and she claimed that she took knowledge of his language from his mind to explain her having previous spoken in some kind of odd wailing noise which was 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, but since Human DNA hadn’t been entered into the force field, they did have a chance to get at the warden. With one Phiadoran per member of the team (baring the Doctor), they eventually managed to get to the warden's base, and the Doctor was able to reprogram two of the warden’s flying sentries to allow him and Turlough to pilot them.

The British soldiers were then forced to race against time back to their ships, as the force field that kept the air in the crater was shutting down with the death of the warden. However, the Doctor, Turlough, and Lytalia became separated form the rest, and only just managed to find their way to the Draco. However, once there they discovered that the helmsman, Stanton, had mutinied, and only a few crewmen were left. The Doctor managed to get the ship up, but he was then kicked out by Stanton’s crew, leaving Turlough alone on the ship- with the Vrall/Lytalia picking them all off. Fortunately, the Doctor was found by Kamelion, and they managed to rescue Turlough before Stanton drained the air out of the Draco to stop the Vrall.

The Doctor, having managed to work out the truth about the Vrall thanks to two facts (that they used Victorian language when they claimed to have taken their language from Turlough and the fact that they couldn’t understand Lytalia’s language when they saved her), attempted to stop them landing on Earth, but was too late. Fortunately, he and Turlough located some weapons on the moon that could hurt the Vrall, and destroyed them all. (PDA: Imperial Moon)