Eighth Doctor's regeneration

"Physician, heal thyself."

- Eighth Doctor's last words according to one source.

Given that there were multiple different incarnations to have regenerated from the Eighth Doctor, (PROSE: The Gallifrey Chronicles, The Tomorrow Windows, etc.) the regeneration of the Eight Doctor was subject to largely differing accounts. The Eighth Doctor either regenerated into a "listless" incarnation, (TV: The Curse of Fatal Death, PROSE: The Tomorrow Windows, The Gallifrey Chronicles, etc.) a "pale, aristocratic" incarnation, (WC: Scream of the Shalka, PROSE: The Tomorrow Windows, The Gallifrey Chronicles, etc.) an incarnation suffering from the guilt of killing of all the Time Lords, (TV: Rose, PROSE: Have You Seen This Man?, The Tomorrow Windows, The Gallifrey Chronicles, Doctor Who and the Time War, etc.) or a forgotten incarnation who renounced the name "Doctor". (TV: The Day of the Doctor, etc.)

Many accounts held that the Eighth Doctor fought in the Last Great Time War, (PROSE: Museum Peace, Osskah, COMIC: The Forgotten, etc.) and so regenerated into the Ninth Doctor after using the Moment to destroy Gallifrey Original; (PROSE: Doctor Who and the Time War) the Ninth Doctor was seen on Earth shortly after, still reeling from the guilt of his actions. (PROSE: Have You Seen This Man?, etc.)

However, other accounts held that the Eighth Doctor had regenerated into the War Doctor with the assistance of the Sisterhood of Karn during the Last Great Time War, having been killed when Cass Fermazzi's gunship crashed on the planet Karn with him aboard. (TV: The Day of the Doctor) This was the eighth regeneration of his first regeneration cycle. (TV: The Time of the Doctor)

Anticipation
On several occasions, the Doctor felt that he would soon regenerate. (PROSE: Museum Peace, The End)

The Nine, the ninth incarnation of another Renegade Time Lord who had himself recently regenerated from the Eight, briefly deceived the Eighth Doctor's companions, Liv Chenka and Helen Sinclair, into believing that he was the Doctor. (AUDIO: The Crucible of Souls)

Regeneration on Karn
When the Eighth Doctor tried and failed to rescue Cass from her crashing ship, he stayed behind and let himself die when it crashed on Karn. While he had died too quickly to trigger a regeneration, the Sisterhood revived him with the Elixir of Life, with Ohila giving him four minutes to choose a specially prepared Elixir to influence his next incarnation. Deciding that the universe no longer needed "a doctor" during the Time War, he chose to become a warrior (TV: The Night of the Doctor). According to one account, this Elixir was a placebo effect to bring out the Doctor's potential as a warrior, and he would have regenerated at the end of the four minutes anyway. (PROSE: The Day of the Doctor)

When the Doctor's regeneration was triggered by the Elixir of Life, he saw his hands glow golden, before being overtaken by the painful process, forcing him to keel over as the golden light overtook him. (TV: The Night of the Doctor)

End of the Time War
to be added

Legacy
The Tenth Doctor remembered that the Eighth Doctor had died alone. (COMIC: The Forgotten)

Other realities
The account of the Eighth Doctor's regeneration in which he did so on Karn, with the assistance of Ohila, also occurred to his counterpart who existed in the Daft Dimension, with two different accounts of events deviating slightly.

By one account, the Doctor took his time to thank past companions and various other individuals who had previously helped him out in menial ways, only to find that the elixir had gone cold, leading him to ask for a fresh one. (COMIC: The Daft Dimension 592)

According to another account, he drank the elixir and commented that it tasted like chicken, leading Ohila to realise she had gotten the elixir mixed up with Sister Ethel's cup-a-soup. (COMIC: The Daft Dimension 499)

Behind the scenes

 * Paul McGann, who portrayed the Eighth Doctor in the telemovie, said that he would have returned to 2005's Series 1 if given the chance, but Russell T Davies did not want to depict a regeneration in Rose with first-time viewers tuning in, who would be unable to identify why the Doctor changed appearances.
 * The Flood was originally going to end with the Eighth Doctor's regeneration, with the BBC approving the plan. However, a condition imposed by the BBC and Russell T Davies (namely, that the Ninth Doctor could not be seen to travel with any companion other than Rose) rendered this unworkable, and the idea was abandoned.
 * Thought was given to a storyline that would have seen Destrii travel with a Doctor who was 'trapped' mid-regeneration, with a flaming head and hidden features that the writers and artists compared to Dormammu of Marvel Comics, but this idea was considered too much effort for too little reward, considering the main purpose of a regeneration storyline was to see how the companions reacted to the new Doctor.