“Harvest of Time”
A particularly interesting detail to note about the Sild is that they are actually The Master’s enemy more than they are The Doctor’s enemy, as he was responsible for locking them away in the first place.
In their natural state, the Sild resemble small glowing seahorses, but have always been shown in what appear to be mechanical silver crabs, each one about the size of a regular crab but with feelers/tentacles on their ends. The Sild are vulnerable to conventional weapons, but their sheer weight of numbers makes them a potent threat, particularly since they can take control of those they attack by latching on to the back of their necks; lacking any concept of the individual, the Sild will sacrifice any numbers necessary to achieve their goal. The Sirl require a few seconds to take control of a subject once they are in the right position, but after this link has been formed any attempts to remove them will kill the host instantly, to the point where even The Doctor had little qualm about running down Sild hosts while driving as they would already be dead. Even a partial connection can be dangerous; a Sild once attempted to take control of the Master, and while The Master resisted it long enough for The Doctor to drive the Sild away, the resulting neurological damage would have eventually killed The Master if he hadn’t been able to find an Infinite Cocoon to repair his damaged body (It was unclear if this attack would have prevented regeneration or not, considering that The Master at this point was implied to have exhausted most of his lives (“The Dark Path” and”The Deadly Assassin”).
The Sild were one of many dangerous races and discoveries locked away in the Consolidator, a massive prison ship created by the Time Lords – albeit after they’d already been sealed up by someone else – to isolate the contents from the rest of the universe as they were believed to be too dangerous to be allowed to roam free even as some Time Lords opposed destroying them due to Article 7 against genocide (The Trial of a Time Lord story”Terror of the Vervoids”) or the possibility that the discoveries could be useful later. Assigned to come up with a solution where their peers had failed, the students who would become The Doctor and The Master worked on the project until the Time Lord that would be The Master had the idea of using a black hole to tear a rift in time and send the Consolidator into the distant future, where the future Time Lords could deal with it (The Doctor declined to have his name put down on the calculations as he questioned the ethics of the assignment). However, when the experiment was actually attempted, the Consolidator was apparently destroyed by a mistake in the calculations when it struck the edge of the black hole, leaving the Time Lords to hush the matter up.
In reality, however, the Consolidator was left drifting in space for centuries after The Master’s calculations proved to be so good it was sent even further forward than anticipated. In this state, the Consolidator was eventually drawn to the planet Praxiledition, which had recently undergone a period of industrial development after the arrival of the Red Queen, a human woman from the distant past who had apparently crash-landed on the planet in her life-pod. Research into the Consolidator continued until the Sild were discovered during a search, releasing them on the rest of the universe as they drew on Praxiledition’s environment to mount their assault.
The Sild became aware of the Master’s presence when he managed to send out a telepathic message for help during his captivity on Earth (“The Dæmons” and”The Sea Devils”). Having been forcibly recruited by a branch of the military to help them design a system that would allow submarines to communicate via neutrinos, The Master attempted to use the system to transmit a signal through time that would allow him to call on another version of himself to aid in his escape, but his message provided the Sild with something to focus on. Using this information, and the resources of the Consolidator, the Sild began to reach out through time, searching for and capturing various versions of the Master (Most likely some were merely possible versions of the Master, given that they numbered in the hundreds, although some of the Masters described did resemble his canonical future selves, such as a disfigured form (“The Deadly Assassin”), a few female versions (“Dark Water/Death in Heaven”), and a clean-shaven man in a suit that reminded The Doctor of a politician (“The Sound of Drums/Last of the Time Lords”). The Silds’ abduction of the Master had the side-effect of creating a time fade that caused everyone on Earth to gradually forget about The Master, apparently based around the length of time that people had spent with him; at first, The Brigadier, Mike Yates, and Benton had difficulty recalling The Master’s name while The Doctor, Jo Grant and the governor of his current prison still clearly remembered him, but towards the end of the crisis only The Doctor could clearly recall his enemy. UNIT were forced to put up posters around the base to remind themselves who The Master was, but The Doctor was still certain that even his memory of the Master would be lost given enough time.
As UNIT was called in to investigate the disappearance of an oil rig, The Doctor determined that the rig had been destroyed by a temporal rupture that literally took part of the rig into a different location in time and space, realising that the same process was sending something to Earth. Although certain that the time-fade and the temporal ruptures were connected, The Doctor was forced to prioritise when the Sild attacked, forcing him to take the TARDIS into The Master’s prison to rescue him (Able to bypass the Time Lords’ current restrictions (“Spearhead from Space”) due to the temporal ruptures being created). Despite The Master’s natural reluctance to depend on The Doctor for anything, the attack of the Sild forced him to accept The Doctor’s help and retreat into the TARDIS. When UNIT was forced to drop a nuclear bomb on the prison to try and stop the Sild acquiring The Master, the two Time Lords found themselves drawn into the far future as the TARDIS dematerialised while the bomb detonated.
Following the signal used by the Sild to its source, The Doctor and The Master found themselves in a vast chamber inside what would later be revealed to be the Consolidator, filled with bodies in stasis chambers. Examining the bodies in the chamber, The Doctor and The Master realised that they were all versions of the Master, each Master linked in an elaborate psychic networkâ with a version of the Master in his current incarnation as the central ‘node’ in this network. Having been taken down to Praxiledition by a robotic servant of the Red Queen, The Doctor and The Master learned the history of the Sild’s awakening, as well as the fact that, by this point in history, the Sild had all but destroyed the rest of the universe. With this knowledge, The Doctor and The Master destroyed the Consolidator in that time before travelling back to an earlier point in history, where The Doctor and The Master were able to heal the neurological damage The Master had sustained when he was attacked by the Sild by using an Infinite Cocoon, a device that could reshape the physiology of anything that entered it so long as the appropriate equations were used (The Red Queen and the Praxileditions had used it to turn some of the worm-like Praxileditions into humans to better explore the Consolidator, but with few successes due to the complexity of the transformation, although The Master was able to successfully program it due to the more basic nature of this change).
In the process, The Doctor realised that the Red Queen was Edwina McCrimmon, the head of the rig that he had visited earlier; she had fallen into a temporal rupture while trying to escape the rig in a life-boat and the trauma of time-travel had addled her memory, as well as her undershooting her target and arriving at the source of the rifts before they started. Despite this discovery, the Red Queen still sent The Master up to the Sild, hoping that she could thus spare her people from the Sild assault, The Doctor accompanying them out of a lack of options. However, after The Master had been connected to the network, he revealed that he had tricked the Sild; using the Infinite Cocoon and his memory of the scan he had taken of the Assembly in the future, The Master had not only repaired the damage the Sild had inflicted on him, but reconstructed his mind so that he would be eminently suited to channel the power of the Assembly, allowing him to take back control of the networked Masters from the Sild and create his own temporal ruptures, tearing through the Sild. Fortunately, his power was limited as the other Masters resented him trying to control them (The Doctor reflecting that, if he couldn’t get on with himself, The Masters were never going to cooperate with each other), the resulting conflict between the various Masters erasing some of the alternate Masters from history and tearing the Consolidator apart, forcing The Doctor and The Master to flee back to Praxiledition and return to the TARDIS.
Although The Doctor was forced to leave The Master behind even after they used the Infinite Cocoon to save Edwina when The Master tried to recover a bomb that had been in the Consolidator’s cargo, The Master later reappeared, claiming that he had found an abandoned Type Forty TARDIS at the end of the universe and used it to return (Although, given The Master’s duplicitous nature and the temporally fragmented nature of the conflict, other possibilities present themselves). The Master was subsequently recaptured, but The Doctor was still satisfied with the final outcome as the Sild had been defeated and The Master’s attempt to acquire more power had been averted.