Tenth Doctor's regeneration

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 '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)

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)