The Garvond, a primitive variation of it was known as the Dark Matrix
“The Dimension Riders”, a primitive variation of it appeared in “Matrix”.
The Garvond only confronted The Doctor once in its purest form, but it was clearly a formidable adversary for him, due to both its temporally active nature allowing it to disrupt the fabric of time itself, and the fact that it only existed because of the Doctor.
According to Gallifreyian legend, the Garvond would manifest from the Matrix, where it was imprisoned by the power of the minds in the Panatropic net while feeding off the minds’ hatred and fear until it managed to escape, its various names, including Garivont and Garavond, were all corrupted versions of the High Gallifreyian word gjara’vont, meaning ‘of darkest thought’. Having learned of the Garvond’s existence, The Doctor believed that he had defeated the Garvond when he erased his own mental print from the Matrix, having learned from various prophecies that his mind would play a part in the Garvond’s escape (Although he described his contribution to it as nothing more than one shareholder in a corporation with many), but an initially-unknown foe subtly changed history so that The Doctor never erased his mental print, resulting in the Garvond being created anyway.
In its ‘natural’ state, the Garvond resembled a human skeleton cloaked in darkness sitting on a flaming throne, served by Time Soldiers, former time-travellers who fell under the Garvond’s influence and whose weapons were literally part of them, bombarded with so much information due to their constant time-travelling that they couldn’t interact with people around them. Seeking to gain power after it came into existence, it manipulated a renegade Time Lord known as the President – after his position as President of St Matthews College, Oxford, in 1993 – into assisting it in assassinating a government minister during a period of ‘crystallized time’ (A period of time in the Vortex where everything was meant to happen in a specific manner), the disruption of such a time period releasing a vast amount of energy as time reshaped itself to accommodate the damage and thus making the Garvond even more powerful as it sought to ravage time itself. The Garvond had selected the President due to his bitter feelings towards Gallifreyan society as it failed to recognise his talents, promising him power over Earth and Gallifrey in return for his help.
Becoming aware of the Garvond’s plans while investigating a time distortion linking Oxford in 1993 to Space Station Q4 in the 24th century, the Seventh Doctor set out to investigate, leaving Benny in Oxford while he and Ace travelled to Q4, where they discovered that the entire crew had apparently aged to death, only to be arrested by the survey ship Icarus that had been sent to investigate the distress call. While examining Q4, the Icarus discovered a message left by The Doctor a week ago, programmed to be sent at that moment, but their inability to accept that Time could be used as a weapon delayed their departure long enough for the Garvond to dispatch its Time Soldiers against the ship. While Ace fled with the crew, The Doctor and Icarus captain Romulus Terrin managed to escape by travelling through the Garvond – The Doctor’s connection to the Garvond allowed him a degree of immunity to the Garvond’s time-manipulating abilities, and Terrin was close enough to The Doctor to share in that protection – only to arrive on Q4 before the original attack, allowing The Doctor to record the previously-discovered message as he realised what they were up against and convinced the crew of the threat they faced.
Meanwhile, Benny attempted to investigate the Oxford end of the anomaly with the aid of Professor James Rafferty – an old friend of the Doctor’s with UNIT contacts – and his student Tom Cheynor, only to be caught by the President when a device that The Doctor had given Benny to find the TARDIS directed her to the President’s ship. Having taken his prisoners to Q4 to capture The Doctor and Terrin, the President revealed his plans, while Ace, alone a week in the future, tried to help the crew fight the Garvond’s Time Soldiers, but failed as they could only be injured by their own weapons due to their fragile connection to time meaning that they weren’t quite there. Although The Doctor was forced to summon his own TARDIS to the President’s ship, the President quickly learned that he had now served his purpose in the Garvond’s plans, having assumed various aliases over the years, he had been unaware that the minister the Garvond would assassinate was actually another of his assumed identities, with the result that he had arranged for his own death, thus giving the Garvond a new paradox to feed on. Returning to the Icarus, The Doctor revealed that the two time periods were connected as one of Tom’s descendants was a member of the Icarus crew, allowing the Garvond to possess Tom and enter the TARDIS, only for The Doctor to reconfigure the internal architecture to give himself and Ace time to find a means of stopping it.
When the Garvond attempted to use the Icarus to create a new paradox by drawing it back in time to cause the disaster that prompted it to travel to Q4 in the first place, the crew were able to destroy the Time Soldiers by modifying Q4’s asteroid deflection shield to turn the Soldiers’ weapons against them, thus averting the destruction of Q4 (Another temporal paradox, as now there had never been a distress signal sent out for Icarus to investigate, but The Doctor considered it a minor one that would be easily resolved) and destroying the Soldiers. The destruction of its soldiers disrupted the Garvond as it attacked The Doctor and Ace in the TARDIS library, allowing for The Doctor to trap it in the transdimensional book, The Worshipful and Ancient Law of Gallifrey, which he then disposed of. His subsequent investigations revealed that the individual responsible for changing the past to prevent The Doctor averting the creation of the Garvond was his old foe the Meddling Monk (“The Time Meddler”), using a captured Chronovore (“The Time Monster”), who sought to keep The Doctor distracted while he aided the Vardans (“The Invasion of Time”) in their invasion of Earth (“No Future”) (The Monk even admitted that the Garvond had never been intended to kill The Doctor, but merely distract him until The Monk was ready to act).
Although the Garvond was erased as an independent threat, the Dark Matrix – the manifestation of the darkest thoughts of all deceased Time Lords – is clearly similar to it in nature, as they both ‘evolved’ from the darkest portions of the Matrix and have the power to warp reality. As the Dark Matrix was shown to be little more than a raw destructive force as opposed to the Garvond, which was capable of speech and rational thought even if it was consumed by its lust for destruction, this suggests that the Dark Matrix is the Garvond without the ‘catalyst’ of the Doctor’s mental print to give it the final push to achieve sentience. When The Doctor’s dark future self, The Valeyard (“The Ultimate Foe”) took control of the Matrix after he was defeated by the Sixth Doctor, he gained access to the Dark Matrix, seeking to use it to reshape reality and give himself a true existence (“Matrix”).
Attempting to gain full control over the Dark Matrix, The Valeyard took it to Earth in his corrupted TARDIS, settling in London in 1888 and committing murders as Jack the Ripper to ‘feed’ the Dark Matrix, also using its influence to corrupt The Doctor’s various incarnations into wraiths who would do his bidding, but his plan was eventually thwarted by the Seventh Doctor (At a point before his encounter with the Garvond), who had learned about the damage that the Dark Matrix would inflict on history during a visit to an alternate 1963 and was able to use the TARDIS telepathic circuits to shield himself from his enemy’s influence. Having tracked down his enemy in 1888, The Doctor provoked the Dark Matrix into trying to escape The Valeyard’s TARDIS by informing it that it was still a trapped servant, its attempt to escape the ship destabilising the interior and causing the TARDIS to collapse in on itself, apparently destroying the Dark Matrix and The Valeyard while undoing the dark future that The Doctor and Ace had witnessed.