Thirteenth Doctor's regeneration

"Right then, Doctor "Whoever-I'm-about-to-be". Tag, you're it."

- The Thirteenth Doctor's last words

Having been mortally wounded by the Qurunx's destruction beam after it was redirected towards her by the Spy Master, the Thirteenth Doctor underwent her regeneration into the Fourteenth Doctor whilst standing on a cliff just outside of the TARDIS. (TV: The Power of the Doctor) This regeneration was part of the Doctor's new regeneration cycle. (AUDIO: Salvation)

Anticipation
After the Flux, Time hinted at the Master's Dalek Plan and warned the Doctor, "you won't outrun me. Your time is heading to its end. Nothing is forever. No regeneration, no life." (TV: The Vanquishers)

After Yasmin Khan expressed her attraction to the Doctor, the Doctor said that she couldn’t get into a relationship because “time always runs out”. The Doctor later asked Yaz to try to live in the present of what they had while they still had it, and made a wish on a skipping stone that it could last forever. (TV: Legend of the Sea Devils)

Now anticipating her regeneration, the Thirteenth Doctor wrote a book about all of her lives, also using material written by other incarnations from the past and her fam, to aid the new Doctor if they "[felt] weird" after regeneration. (PROSE: A Short History of Everyone)

Causes
At the end of the Master's Dalek Plan, fatally wounded the Thirteenth Doctor as revenge for the reversal of her forced regeneration. (TV: The Power of the Doctor)

Regenerative process
Coming to a cliff, the Thirteenth Doctor regenerated. This regeneration involved large spouts of golden regeneration energy which had an electrical crackle. Unusually, her successor had taken on the appearance of their tenth incarnation. (TV: The Power of the Doctor)

Aftermath
Immediately after the regeneration, which he described as beautiful, (PROSE: ) the Fourteenth Doctor reflected on how he should've been "somebody new", not "somebody [he'd] been before", (PROSE: ) before entering the TARDIS, writing about the experience in a letter to the readers of an annual, (PROSE: ) and answering a distress call from the TARDIS which led him to the Dalek Dome, becoming embroiled in the 2323 Dalek invasion of Earth. (COMIC: ) He was then briefly diverted to Skaro during the genesis of the Daleks, observing that he was a "really brilliant woman" only 60 minutes prior. (TV: )

After bumping into Donna Noble, the Tenth Doctor's former companion, the Fourteenth Doctor, aware of the risks of Donna remembering him after her transformation into the DoctorDonna, reflected on his regeneration into an "old face" and confided his fears to Shirley Anne Bingham. (TV: )

Theoretical regenerations
During the Thirteenth Doctor's life, the possibility of regeneration was discussed for several situations prior to her actual regeneration.

While facing being shot by guards of the Good Doctor, she said any attempt on her life would result in a "miracle". (PROSE: The Good Doctor)

Trying to generate enough energy, the Thirteenth Doctor once considered regenerating and using residual artron energy. (PROSE: The Maze of Doom)

During the Redaction, the Doctor remarked to Cleo Proctor that she would stand up again if she "clobber[ed]" her, remarking that she would regenerate "Possibly several times" and then "who can say?" as to how many times. (AUDIO: Salvation)

Behind the scenes
A lengthy segment in DWM 584 details the creation of the Thirteenth Doctor's regeneration sequence. Power of the Doctor 's primary director, Jamie Magnus Stone, shot drone footage of Durdle Door; visual effects designer Ben Pickles augmented the footage, adding a CG sun and lighting to a drone shot filmed just before sunrise. To make the lighting match, Pickles had to make an entirely CG 3D-scanned cliff for all of Tennant's shots; the cliff was also digitally altered to provide a flat surface for the TARDIS to sit on. No filming with actors actually occurred atop Durdle Door.

Whittaker's half of the sequence was filmed at a green screen soundstage at Roath Lock Studios on 13 October 2021, with Magnus Stone directing. Filming for Tennant's half took place seven months later - on 13 May 2022 - at a soundstage at Wolf Studios, with director Rachel Talalay filling in for Magnus Stone. While filming for the latter section, the crew used a monitor to superimpose and frame-match the postures of the actors.