According to the Doctor, vampire legends can be found on almost every inhabited planet in the universe. The ubiquity of such stories strongly suggests a basis in fact - and, indeed, such a basis exists.
In the early years of Time Lord society, when even Rassilon was young, multitudes of giant vampires appeared out of nowhere and swarmed all over the universe. These vampires were so strong that one vampire could suck the life out of an entire planet. Determined to rid the universe of this vampire army, the Time Lords hunted them down in a war so long and so bloody that they were sickened of violence forever. All the giant vampires were destroyed - except for one, which vanished into thin air.
This battle was recorded in the Record of Rassilon: "So powerful were the bodies of these great creatures, and so fiercely did they cling to life, that they were impossible to kill, save by the use of bowships. Yet slain they all were, and to the last one, by the Lords of Time - the Lords of Time destroying them utterly. However, when the bodies were counted, the King Vampire, mightiest and most malevolent of all, had vanished, even to his shadow, from time and space. Hence it is the directive of Rassilon that any Time Lord who comes upon this enemy of our people and of all living things, shall use all his efforts to destroy him, even at the cost of his own life."
The King Vampire had made the transition into Exo-Space, otherwise known as E-Space, a much smaller universe than our own (Normal or N-Space). This seems to be an ability that it inherently possessed, as it is said to also know the way out of E-Space. Badly wounded, the Giant Vampire settled on a planet in E-Space - probably uninhabited at the time - and then used its telepathic powers to locate a spaceship in N-Space, the exploration vessel Hydrax en route from Earth to Beta 2 in the Perugellis sector. The main crew of the Hydrax were captain Miles Sharkey, navigational officer Lauren MacMillan and science officer Anthony O'Connor. O'Connor apparently had some latent telepathy of his own, for it was through his mind that the Great Vampire spoke to the others, and summoned the Hydrax to E-Space, perhaps directing them through a CVE.
The Hydrax officers, having been turned by the Great Vampire into vampires themselves, proceeded to feed and nourish him. 1000 years and 20 generations passed before he was healed and ready to arise. By this point, the planet had developed an early medieval society centered around the Hydrax, which had become known as the Tower. The Hydrax officers, who ruled the planet, also underwent a natural name change as predicted by linguistic rules - Sharkey became King Zargo, MacMillan his Queen Camilla, and O'Connor the councillor Aukon. Known as the Three Who Rule, they stayed in the Tower while the peasants - perhaps descendants of the original Hydrax crew - toiled in the fields outside the only village on the planet.
The Lords wanted to keep the society stagnated, so all science, even all knowledge, was forbidden. The penalty for such curiosity was death. Naturally, the village contained no schools - children were sent to work in the fields at a young age, and continued doing so all their lives. The only children who escaped this fate were the ones taken during the Selection, when the Lords or their guards would choose particular youths to serve them in the Tower. Instead of moving forward, this society actually moved backward, keeping it in a permanent state of decay. Those who spoke out against the Lords and their rule died silently in the night, so most villagers resigned themselves to their monotonous lives - after all, the Lords protected them from the wasting.
Of the Three Who Rule, Zargo and Camilla are undoubtedly the most public figures - as the King and Queen, they appear to be the most powerful, with Aukon more of a silent partner. But what the peasants do not realize is that Aukon, although silent, is by far the strongest of the three. Zargo is often beset by uncertainty and fear, and Camilla is ruled by her hunger. But Aukon maintains his sense of inner calm and his foresight; no doubt this is aided by his telepathic link with the Great One, with whom Aukon frequently communicates. This link seems to have augmented Aukon's telepathic powers; he can exercise a hypnotic influence over people using his eyes, even to the point where he is able to control their bodies, and he also communicates with the resident bats of the planet. These are the servants of the Great Vampire, and Aukon uses them to be his eyes and ears. What they feed on apparently also nourishes the Great Vampire. And unlike regular Earth bats, these bats are vicious - during their nightly swarming, they will attack and bite wandering humans.
Zargo and Camilla have redecorated the Hydrax so that it looks like a medieval castle, down to a throne room that disguises the previous control room. But the technology still exists - an inspection hatch leads upward from the control room to three arrow-class scout ships which can detach from the main vessel for local exploration. Moving downward from this point leads first to a room full of pallets on which the Lords keep the drained bodies of their victims - probably some of the villagers who were taken during the Selection. Presumably the bodies are usually disposed of discreetly, although as the vampires' moment of triumph draws near, they drop the pretense and sometimes return the dead bodies to their relatives. The blood drained from these bodies is kept in the Hydrax fuel tanks, from which pipes carry it down to a large cavern underneath the ship, known as the "Resting Place". The pipes, which serve as a feeding system, nourish the Great Vampire where its body lies underneath the cavern, the ground rising and falling in time with its breath. Its heartbeat echoes throughout the chamber and even through the ship.
The Three Who Rule are constantly working toward their Great Purpose - to heal the Great Vampire until he is well and can arise; for at the Time of Arising, his servants will swarm into N-Space and drain the life from the multitudes of planets that can be found there. In order to prepare for this, the vampires need to select certain humans to turn - they call this an "initiation", and the humans they select are their Chosen Ones. The vampires consider that humans can serve only two purposes: either they can join the Great Vampire or they can feed him.
Vampires fear sunlight (the implication is that sunlight does not kill these vampires, although there are no windows in the rooms they frequent in the Hydrax), running water, and certain herbs. They are so strong that they can only be killed by beheading or a stake through the heart. Metal does not hurt them, so a dagger through the heart is merely an annoyance. A trace of humanity does remain in them - they still feel emotions like fear. They also require sleep, although there does not seem to be any particular place where they must rest. But when they wake, they wake hungry. Only the blood of the living will do - the blood of the dead is stale and flat. Romana theorizes that the transformation into a vampire must take some time, but there is no evidence either for or against this theory. Vampires are extremely strong, and they also hiss at their intended victims. The giant vampires are quite a bit different - for one thing, a mere stake is not enough. The Record of Rassilon said, "Energy weapons were useless, because the monsters absorbed and transmuted the energy, using it to become stronger. Therefore Rassilon ordered the construction of bowships, swift vessels that fired a mighty bolt of steel to transfix the monsters through the heart, for only if his heart be utterly destroyed will the vampire die." The reason for this is that their cardiovascular system is extremely efficient; so, it seems that either wood or metal will do the job for a giant vampire, so long as it destroys the heart. Physically, giant vampires are much more batlike than the humans they turn; while the head is vaguely humanoid, the ears are batlike and there is the suggestion of wings beneath their clawed hands.
Just at the Time of Arising for the Great Vampire, the Doctor programmed one of the Hydrax's scout ships to launch straight up into the air, then turn and come straight down. The scout ship pierced the Great Vampire's heart just as its arm was emerging from the ground, killing it. Within moments, its vampire servants slowly aged into smoking skeletons and died.
