Tenth Doctor's regeneration
I don't want to go.
The Tenth Doctor, who had previously regenerated and kept the same face during the Planetary Relocation Incident, (TV: Journey's End) met his regeneration into the Eleventh Doctor as a result of radiation poisoning intentionally incurred in order to save the life of Wilfred Mott after thwarting Rassilon's Final Sanction. (TV: The End of Time) By later accounts, this was the twelfth and final regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)
History
Anticipation
After the Doctor and Donna Noble aided in the revolution of the Ood in 4126, Ood Sigma ominously warned the Doctor that "[he thinks his] song must end soon". (TV: Planet of the Ood)
During the Planetary Relocation Incident, the Doctor was fatally shot down by a single Dalek, triggering a regeneration. However, he was able to undergo the process without changing his appearance, by siphoning the remaining Artron energy into his severed hand after it had healed him. This later led to the creation of the Meta-Crisis Doctor. (TV: The Stolen Earth/Journey's End)
When he met Jackson Lake, a 19th century man who believed himself to be the Doctor after having picked up an infostamp, the Doctor initially assumed Jackson to be either his next incarnation or the "next but one". (TV: The Next Doctor) By a later account, however, he assumed the Twelfth Doctor to be his next and final incarnation, knowing his next regeneration to be his last. (COMIC: Vortex Butterflies) In an alternate timeline, in an encounter between him and his two immediate successors, the Tenth Doctor was skeptical of his twelfth incarnation's identity, believing "this man [could not] exist. Shouldn't exist!" (COMIC: Four Doctors)
After he brought The 200 London bus and its passengers back through the wormhole from San Helios, Carmen, a passenger with precognitive psychic abilities, echoed the words of Ood Sigma, telling the Doctor that his song was ending, adding:
"It is returning. It is returning through the dark, and then Doctor...oh but then... he will knock four times."
Shortly before his regeneration, the Tenth Doctor saw a solitary pale Watcher observing him; shortly after the TARDIS dematerialised, the Watcher also disappeared. (PROSE: The Advent of Fear)
Behind the scenes
Four different takes of the Doctor's final words were recorded, in which the emotion and grief was pushed further with each delivery. Said deliveries ranged from blunt and almost emotionless, to the Doctor being on the verge of tears. (CON: Allons-y!)
