Forced regeneration

A forced regeneration was an instance of regeneration that was imposed on an individual when they did not choose or need to regenerate due to injury, usually as a form of punishment, forcibly selecting a new form for the victim.

Cultural significance
claimed to the Thirteenth Doctor that it was the "ultimate sanction" for breaking the Time Lords' laws, (TV: The Power of the Doctor) although other accounts suggested the harshest sentence in Gallifreyan law was to have one's life-stream reversed such that they had never existed. (PROSE: Doctor Who and the Terror of the Autons)

Instances
In another reality, Narvin identified the Fourth Doctor's regeneration into the Warrior as a forced regeneration. (AUDIO: Aftershocks)

suggested to the Thirteenth Doctor that she might have been subjected to forced regeneration multiple times in her past, alluding to (TV: The Power of the Doctor) his own discovery that in the course of her research, Tecteun had forced at least five regenerations on the Timeless Child, who would later become the Doctor. However, the records found in the Matrix by the Master did not specify how the Child's unwilling regenerations occurred or that Tecteun had control of the forms adopted by the Child as a result. (TV: The Timeless Children)

The Second Doctor's sentence for his trial for stealing the TARDIS and breaking the non-interference policy was to forcibly take a new face and be exiled to Earth. They gave him a chance to pick one of multiple faces they had prepared for him, but he dismissed them all, leading to them forcing yet another appearance upon him. (TV: The War Games) Though the Celestial Intervention Agency was able to delay the execution of this sentence for some time, (PROSE: World Game) the Time Lords eventually sent animated scarecrows to carry it out. (COMIC: The Night Walkers) The TARDIS was sent on a programmed trip to Oxley Woods and the Doctor had his knowledge of how to operate or repair it blocked off. (TV Spearhead from Space, The Claws of Axos) In the Inferno universe, the Second Doctor was similarly forced to regenerate but chose one of the faces initially suggested to him, the thin man, becoming the Leader. (PROSE: Timewyrm: Revelation)

As part of the "Project Lazarus", initiated by the Forge's leader Nimrod, the Sixth Doctor was captured and experimented upon to study the regeneration process. Nimrod almost succeeded in making the Doctor regenerate, but he was interrupted by Cassie, who got her previously altered memories back to normal thanks to Evelyn Smythe. The Doctor escaped, but Nimrod had his DNA samples saved and created a large amount of the Doctor's clones and killed several of them to make them regenerate, but the clones lacked that ability. Nimrod's plan was foiled by the Seventh Doctor, who told the truth to one of the clones, and who activated the facility's self-destruct mechanism. (AUDIO: Project Lazarus)

During the War in Heaven, the House Military "primed" their soldiers, both Homeworlders and recruited from the "lesser species", so that each of their regenerations would result in them becoming more inhuman and war-machine-like, taking control of their new forms away from them. By the Third Wave, forced regen missions, where an entire squad was forced to regenerate into bodies adapted to the specific terrain before the mission started, had become a reality. The forced regen missions were viewed as scandalous by the troops themselves, and widened the schism between House Military troops and the Homeworld. (PROSE: The Book of the War)

As part of his so-called "Master's Dalek Plan", captured the Thirteenth Doctor and forced her not only to regenerate but to do so into him. The result of the Thirteenth Doctor's forced regeneration was a new incarnation with the appearance and personality of the Master, while the Thirteenth Doctor found herself within their mind at the Edge of Existence, where she met the Guardians of the Edge. They confirmed to her that because she had not yet passed the Edge, the forced regeneration was incomplete, and that post-regenerative trauma to the body was more likely. The Holo-Doctor, who was knowledgeable about the concept of forced regeneration, enlisted the help of Yasmin Khan and Inston-Vee Vinder, and together they were able to reverse the forced regeneration, which the restored Thirteenth Doctor called a "forced degeneration". The Master was returned to his original body, which was severely weakened. (TV: The Power of the Doctor)