An insane Time Lord and old friend turned archenemy of the Doctor, the Master used hypnotism and deception to ensure his election as Prime Minister of the United Kingdom under the human alias of "Harold Saxon", which was his first step towards conquering the Earth and creating a new Gallifrey from it. He is known for being unique amongst other incarnations of The Master due to his genuine hatred and disgust of The Doctor where other incarnations can at least tolerate or enjoy his presence, either for fun or due to lingering feelings of friendship towards their old friend. He is also one of the most unhinged incarnations to date, likely due to the "never-ending drumbeat" he hears in his head. Unlike other incarnations in the Revival Series, the "Harold Saxon" Master is an unfathomably cruel, selfish psychopath with no redeeming qualities. After successfully conquering Earth, the Master orders the population to be decimated for no other reason than to satisfy his own violent impulses. He was extremely vain and narcissistic, with the Tenth Doctor noting that he would never destroy himself, even if he could destroy the Earth with him. During the Year That Never Was, he had monuments of himself built all over Earth, and, according to Martha Jones, had even sculptured himself onto Mount Rushmore. He also used the Immortality Gate to turn the human race into duplicates of himself, which he dubbed the "Master Race", and also threatened to do the same to the Time Lords, even asserting that Rassilon "[would] [look] better as [him]." "Harold Saxon"'s vanity was so vast that when the Tenth Doctor forgave him for his actions, the Master collapsed and wept out of shame. After he expressed revulsion at being "kept" by the Doctor, the Master was shot by Lucy and decided not to regenerate and die to spite the Doctor. He even killed Missy, his future incarnation, when she decided to help the Twelfth Doctor. "Harold Saxon" is the 18th Incarnation of the Master. The Master has powers and skills like that of any Timelord but additionally, he has the abilities of Genius-level intelligence, the ability to regenerate, the use of a laser screwdriver, the loyalty of the Toclafane, perception filter, hypnotism, disguise, and natural charm and charisma. For a bit of time temporarily, he even had the ability to shoot lightnight out of his hands. He is a known renegade criminal of Gallifrey, having committed crimes of Genocide, Theft, Breaking and entering, Abuse of power, Mass murder, Torture, Assault, Subjecting people to a fate worse than death, Incrimination, War crimes, Kidnapping, and much more.

Regenerating into a new body, an incarnation of the Master who once gleefully declared himself a "Spy Master" discovered that the entire Time Lord history had been "built on the lie" of the Timeless Child, as well as the true origin of the Doctor, in the Matrix. Embittered by his discoveries, and lashing out from the belief he was indeed inferior to the Doctor, the Master took his revenge on Gallifrey, leaving it in ruin. The Master used his Tissue Compression Eliminator to kill a newly-recruited MI6 agent on his first day, and took his place. He was put to work as an analyst known as "Horizon Watcher", and given the codename "O" as a joke on how his superior, "C", would react to his presence. During his time at MI6, the Master crossed paths with the Doctor prior to her thirteenth incarnation. Not recognising the Master, the Doctor stayed in contact with "O" via text messages. "O" was eventually sacked by C sometime prior to 2020, and the Master went into hiding in Australia, using his TARDIS as a hideout in the form of a house. Wanting to inform the Thirteenth Doctor of his discovery, the Master worked with a race called the Kasaavin and tech billionaire Daniel Barton in a plot to wipe out humanity. His plan was thwarted at the last second by the Doctor, and the Master was imprisoned in the Kasaavins' realm. The Master eventually escaped and returned to reveal the truth about the Timeless Child to the Doctor and build an army of CyberMasters from the remains of the Time Lords he had killed. His plot was thwarted when Ko Sharmus detonated the death particle on Gallifrey, wiping out whatever organic life remained on the planet, though the Master and his CyberMasters managed to escape. Wanting to ruin the Doctor's reputation, the Master enacted his Dalek Plan where he posed as Grigori Rasputin, ultimately stealing the Doctor's body to become the Doctor himself. However, the Doctor was able to reclaim her body with help from her "extended fam", leaving the Master stuck back in his now damaged body, though he was able to mortally wound the Doctor before he apparently expired. Believing "a little chaos [to be] a wonderful thing", the Spy Master could fall into unpredictable fits of rage, stating he could "relate" to a short-fused bomb to the point that he once attempted to strangle the Thirteenth Doctor with his bare hands. After taking a crowd hostage, he spontaneously focused on a cowering woman and demanded to know if she had moved, only to apologise for his "mistake" before killing her with the TCE. The Master once voiced a belief that nothing could "calm all [his] rage", though the Doctor countered her belief that he was actually "scared of everything". The Spy Master enjoyed playing long games, like tricking the Doctor into believing he was "O", gleefully clapping his hands at the Doctor's inability to deduce who he was despite giving clues and expressing he had had "a lot of fun" when she finally realised he had fooled her. While not above working with others to achieve his goals, the Master did not see his partners as equals, but as assets to use for his own gain and dismiss once they had served their purpose. His arrogance was so profound that he did not even attempt to conceal his TARDIS in 1943 Paris, and felt he was justified in destroying Gallifrey when he learnt that the Time Lords owed their existence to the Doctor, himself included. Believing he had no "better nature", and looking down on Missy deciding to better herself, the Spy Master had a passion for killing others, remarking how it gave him a "buzz", and that it felt right in his hearts to do so. His disregard for life was so great that he even threatened to kill to prove a point. His passion for death, when infused with anger, was reawakened in him when he discovered the lies about the Timeless Child that the Time Lords had committed, driving him to commit genocide back on Gallifrey. Despite not wanting her as his enemy again, the Master loved playing mind games on the Doctor and treating her as an inferior, having her kneel before him when