Thirteenth regeneration

The Time Lord regeneration cycle allowed for twelve regenerations and thirteen incarnations; (TV: The Deadly Assassin) a Time Lord could attempt to trigger a thirteenth regeneration, though this would usually result in their final death. (PROSE: Divided Loyalties)

Indeed, despite not being able to regenerate any further, a Time Lord in their final incarnation retained a small amount of regeneration energy; just enough to mend broken bones once. (TV: The Angels Take Manhattan) Time Lords could will themselves to die by attempting to regenerate when they had no more regenerations left to use, as Azmael who was hosting the evil consciousness of Mestor, chose to do so to prevent it from escaping. (TV: The Twin Dilemma) The Eleventh Doctor likewise threatened to use regeneration to hold off Mr Clever from taking over his mind, fully aware that he had expended his first regeneration cycle's allotted lives. (TV: Nightmare in Silver) A Time Lord on their last regeneration could will themselves to effect a pseudo-regeneration which would cause their body to vanish into thin air. (TV: Shada)

If they attempted to regenerate in earnest beyond their thirteenth and final body, a Time Lord's flesh could break down into degenerate matter and then into random molecules, as was the case for one particular Time Lord. (COMIC: The World Shapers) The Thirteenth Doctor believed that attempting to regenerate "one time too many" was what had once caused the Master to degenerate into (PROSE: The Doctor vs the Master) a barely-alive, emaciated, ambulatory cadaver, (TV: The Deadly Assassin, The Keeper of Traken) although other accounts suggested that this skeletal form was instead simply the Master's thirteenth incarnation after sustaining debilitating injury, (AUDIO: The Two Masters) or indeed an incarnation of the Master who had simply been injured in a fashion that took away his ability to regenerate. (PROSE: Legacy of the Daleks)

The Decayed Master, who claimed to be nearing "the end of his twelfth regeneration", (TV: The Keeper of Traken) was understood by Nyssa and Tegan Jovanka from what the Doctor told them to have used the Source of Traken to facilitate a "fateful thirteenth regeneration". In order to do so, however, the Master required "extra material, a whole new body which he could merge with, absorb and take on". The Master thus possessed the body of Tremas, a Trakenite. (PROSE: Divided Loyalties) This process killed Tremas, (TV: Logopolis) whilst leaving the Master in a younger, rejuvenated version of Tremas' body, (PROSE: Divided Loyalties) becoming a distinct, new incarnation. (AUDIO: Masterful) The Master himself, however, when asked by the Third Doctor if his appearance was "another regeneration", answered "not exactly". (TV: The Five Doctors)

According to one account, the Tremas Master claimed that the Valeyard was the Doctor's "penultimate reincarnation… somewhere between [their] twelfth and thirteenth regenerations". (PROSE: The Ultimate Foe)

A successful thirteenth regeneration could be achieved if a Time Lord was provided with additional regeneration energy, as happened when the Time Lords did so to the aging Eleventh Doctor during the Siege of Trenzalore. The Doctor acknowledged that his "regeneration number thirteen" was "breaking some serious science", proceeding to use the energy he had been given to destroy the Dalek forces above the planet Trenzalore, thus ending the conflict. He then underwent a "reset" which restored his youth in his final moments, saying goodbye to Clara Oswald before finally changing into the Twelfth Doctor, the first of "a whole new regeneration cycle". (TV: The Time of the Doctor)

Behind the scenes

 * The original second volume of The Doctor Who Programme Guide (1981) stated that the Master had used Tremas's body for his "thirteenth regeneration", implicitly designating the resulting "Tremas Master" as the Master's fourteenth incarnation. This would be contradicted in The Five Doctors, in which the Master admits to the Third Doctor that his new appearance is "not exactly" a regeneration.