The Master's Dalek Plan

The Thirteenth Doctor's forced regeneration was imposed on her by as part of his so-called "Master's Dalek Plan" to ruin the Doctor's reputation. However, it was reversed by Yaz with the assistance of the Holo-Doctor. (TV: The Power of the Doctor)

Creation
Following his creation of the CyberMasters and confrontation of the Thirteenth Doctor on Gallifrey, (TV: The Timeless Children) the Master sought to sully the Doctor's title throughout the universe by hijacking her body and wreaking havoc in her name. To achieve this, he enlisted the help of the Daleks and CyberMasters, winning their support by promising to remove their common enemy. The Master also took Rasputin's place in history and hypnotised Tsar Nicholas II into allowing him to use the Winter Palace for his operations, such as the setup of connected regeneration chambers from Gallifrey.

Using the power of the Qurunx, the Master ultimately succeeded in forcing the Thirteenth Doctor into the chambers and triggering her forced regeneration, hijacking the Doctor's body while leaving his own body motionless in the chamber. (TV: The Power of the Doctor)

Activities
Following the forced regeneration, the Doctor's new body had the Spy Master's appearance and personality, and he forced Yaz to be his companion by threatening her with the Tissue Compression Eliminator. Traveling throughout the universe in the Doctor's TARDIS, he caused conflicts and destruction in the Doctor's name.

"I'm going to make the Doctor a byword for fear, pain and destruction, so when people hear that name in future, they quake in fear."

- TV: The Power of the Doctor

From a moon, he watched two planets destroy each other as a result of his plots, and he called out to a visiting hover camera that "I am the Doctor. And I caused this." However, Yaz pushed him out of the TARDIS and fled. He remained on the planet, playing a slow tune on the Doctor's recorder.

Yaz eventually returned, feigning regret for abandoning him. Together they travelled back to the Winter Palace, where he gave the orders for the Daleks and Cybermen to begin their invasion of Earth. (TV: The Power of the Doctor)

Reversal
While the new Fourteenth Doctor went off in a new regeneration of her body, the Thirteenth Doctor remained in their mind, refusing to pass over the Edge of Existence to be incorporated into the new Doctor's memories. While she discussed her situation with the Guardians of the Edge, her AI hologram assisted the Doctor's former companions, including Tegan Jovanka and Ace, to thwart the CyberMasters' and Daleks' invasion plans.

Finally, Yasmin Khan and Inston-Vee Vinder cornered the new Doctor and forced him back into the chamber, using the CyberMasters' regeneration energy to cause a "forced degeneration" that brought back the Thirteenth Doctor and forced the Master back into his body, mortally weaking him as a result. (TV: The Power of the Doctor)

Appearance
Following the forced regeneration, the new incarnation was identical in appearance to the Spy Master, and shortly after regenerating chose a new outfit in the TARDIS wardrobe that incorporated elements of several previous incarnations' costumes, including the Thirteenth Doctor's coat, the Seventh Doctor's pullover, the Fourth Doctor's scarf, the Fifth Doctor's celery and the Twelfth Doctor's checked trousers. He also carried the Second Doctor's recorder. (TV: The Power of the Doctor)

Personality
As this incarnation was the Master in control of the Doctor's body, he shared many of the same quirks, traits and psychopathic tendencies of the Spy Master (TV: Spyfall, etc). This forced incarnation was shown to have an explosive temper much like the Spy Master, blowing up at Yasmin Khan whenever she disagreed or fought against anything he said. He was also determined to carry out the Master's plan of besmirching the Doctor's name and ruining all the good they had done, also showing none of the love and respect the Doctor has for Earth when he gives the Daleks and the Cybermen the all clear to begin invading the planet, as well as proudly declaring himself as the Doctor when showing off all the chaos he had caused (TV: The Power of the Doctor).

Behind the scenes
Through this forced regeneration Sacha Dhawan is the first actor to have portrayed both the Master and the Doctor within the same story, and as this forced incarnation came after the Thirteenth Doctor, Sacha Dhawan technically portrayed a possible version of the Fourteenth Doctor, making it the first time an incarnation of the Doctor has had two different actors portraying them onscreen without an actor standing in for the actual actor, while Sylvester McCoy and Paul McGann acted as stand ins for the Sixth and War Doctors respectfully while being known for their own incarnations of the Doctor.