|The Tomb of Valdemar
Name: Valdemar; essentially possessed Huvan
Time of Origin: Originally active in the distant past; encountered The Doctor at some unspecified date in the future.
Romana; K9 was left in the TARDIS after contact with Valdemar’s energy damaged his systems.
History: Although Valdemar itself was never The Doctor's enemy, it nevertheless proved to be the catalyst for a very dangerous threat that The Doctor had to overcome, to say nothing of being a figure of historical interest.
According to legend, Valdemar was one of the most powerful of the mysterious Old Ones, some of the first beings to ever evolve in the universe, possessing the power to destroy the universe, being a black mass of life that consumed and destroyed all it touched, supposedly released onto the universe by the Old Ones when they penetrated the higher dimensions (Later discoveries suggest that the Old Ones were trying to recapture the unity that once existed before life fragmented into individuality). The other Old Ones were said to have banished Valdemar from the universe on the planet Ashkellia - possessing an atmosphere of liquid sulphuric acid clouds, and a mean surface temperature above six hundred degrees Celsius - before they vanished forever, but vague legends remained even after the Old Ones died out, although even the Time Lords did not know precisely what Valdemar was, with The Doctor and The Master only encountering vague stories about Valdemar when they carried out research in the Matrix while at the Academy.
The existence of Valdemar was eventually rediscovered when aspiring novelist Miranda Pelham heard rumours of his existence during her post-graduation trip around the universe, setting out to record and chronicle everything she could find about Valdemar. She eventually tracked Valdemar to Ashkellia, the planet now covered in toxic gas that served as a massive particle accelerator for the Palace of the Old Ones. The Palace itself was a high-tech building that possessed a self-maintaining, self-regenerating system, although it was now stuck on low power status without a psychic to control it. This research attracted the attention of self-proclaimed magus Paul Neville, who received funding for his expedition to find Valdemar's resting place from the displaced aristocrats (At this point in history, Earth was ruled by the New Protectorate, described by The Doctor as 'the rigours of Puritanism applied to a purely materialistic philosophy' with people forbidden to believe in anything other than the world they now lived in, although The Doctor dismissed it as a temporary period that would be over after a couple of centuries), Neville publically claiming that he sought to take the knowledge and science of the Old Ones back to Earth and have his old titles restored.
After Neville found Valdemar's tomb and the palace of the Old Ones - essentially the doorway to the higher dimensions where Valdemar was contained - the Fourth Doctor was drawn to the planet while he was searching for the segments of the Key to Time ("The Ribos Operation"); the energy released by Neville's attempt to access Valdemar's other-dimensional prison disrupted the tracer that The Doctor was using to find the segments, forcing him to travel to Ashkellia to investigate the source of the disruption so that he could continue his mission. Learning about Neville's presence after he and Romana saved Pelham from an attack by her mutated lover when her attempt to access the tomb disrupted him on the cellular level, The Doctor was taken to the castle so that Neville could ask for his help. While exploring the castle, Romana encountered a group of teenage children staying with Neville - the children of the defunct aristocracy who had funded Neville's work - but the most shocking discovery was her encounter with Huvan, a boy seemingly in the middle of the most stereotypically horrific version of puberty - to the point that his face was covered in pimples and he wrote excessively angst-written poetry, particularly after he met Romana - who arrogantly revealed that he was actually thirty-four years old. Neville claimed that Huvan's warped physical age was the result of him being genetically altered to save him from a degenerative disease that would have killed him if he developed past puberty, but his motives were far more sinister; Huvan had an already -high natural psychic potential before he ever met Neville, and by altering his physiology to trap him at the emotionally and physiologically turbulent time of puberty, Neville had elevated Huvan's psychic levels far beyond the norm.
Believing that Neville would be unable to actually use the palace to awaken Valdemar without a psychic to control it, The Doctor helped him activate it by triggering a psychic 'alarm' to 'wake' the palace up so that Neville would let them go, unaware until it was too late that Neville already had the required psychic controller in Huvan. With Neville having prepared Huvan to become Valdemar's new vessel, The Doctor was forced to leave Romana to keep Huvan calm as he adapted to his powers after the ritual while The Doctor and Miranda searched the palace (Although Romana was unable to stop Huvan subconsciously mutating the other teens into animals in revenge for his past treatment). The Doctor was able to find a formula for a compound that would protect the drinker from the psychological damage they would normally experience when exposed to the energies of the higher dimensions by preventing their bodies manifesting the new sensory organs created by its energy, but he was only able to provide the antidote to himself and Miranda before Neville destroyed the rest of the samples and dropped them out of the palace.
However, instead of being destroyed by Ashkellia's atmosphere, The Doctor and Miranda were instead captured by Neville's nemesis, Robert Hopkins, who had been forcing Miranda to spy on Neville for him - with her and The Doctor facing imminent death, Miranda was forced to activate a teleport bracelet she had been wearing to take her and The Doctor to Hopkins's ship, despite Hopkins wanting her to save it for when she was in contact with Neville - but The Doctor was able to convince Hopkins to let him and Miranda return to the planet to find Neville (Albeit to warn him against potential traps rather than because Hopkins believed The Doctor's warnings about Valdemar). As the palace began to collapse as the higher dimensions intruded on reality, barely kept in check by Huvan's growing power, The Doctor was able to escape Hopkins' guard and transmat down to the tomb with Miranda, but Hopkins swiftly followed him, resulting in him and Neville clashing as they fell into the higher dimensions, the two enemies being merged into a single entity that would continue to tear at times for the rest of its existence due to their absolute hatred of each other.
Unfortunately, while Neville had lost control of Huvan and most of his forces were now dead, Huvan had taken on so much power that his ego had elevated accordingly, prompting him to decide to unmake the universe and revert it to its 'natural' state even without Neville's influence. However, as Huvan entered the gate to Valdemar's prison, The Doctor realised that there must be some other catalyst left to trigger as the higher dimensions hadn't completely entered our plane of existence yet. With this in mind, The Doctor and Miranda following Huvan through the gate to discover the true Valdemar, a massive green and purple creature, The Doctor concluding that Valdemar was the last of the Old Ones who had become a conduit for the higher dimensions due to the experiments of its people, although said power had now been at least partly transferred to Huvan. Having convinced Huvan to remove his new control of Romana to see how she would react to his plans when in control of herself - her freedom aided by the influence of the not-totally-sleeping Valdemar - The Doctor bypassed Huvan's initial outraged reaction to Romana's rejection by revealing Valdemar's last secret; Valdemar was sealed away by Valdemar himself, the Old One having recognised that there was no point in using his power to destroy everything. Acknowledging The Doctor and Romana's wisdom that, even with all his power, he would still never be able to fulfil his single greatest wish of being liked for who he really was, Huvan used his powers to erase his own memory and powers, The Doctor and Romana subsequently taking him off to another planet where his new identity would have the chance to grow and develop as he should have done, while Miranda stayed with Valdemar to learn all that it knew (Huvan's new identity was apparently later taken on as a companion by a future version of Romana, initially regenerated into a form resembling Miranda Pelham before she died of old age and assumed a new body, but consider Gallifrey's current status ("The Day of The Doctor"), it is uncertain if this will ever come to pass).