Time Lord Victorious

The "Time Lord Victorious" was a title the Tenth Doctor briefly claimed due to his status as the "winner" of the Last Great Time War.

A day to come
When the Fourth Doctor encountered a damaged Dalek Drone on the Starship Future, it told him that one of his future selves would turn into everything he opposed by becoming the Time Lord Victorious. The Doctor refused to believe it, but the Dalek teased that "out of any good [done] must [also] come something evil". (AUDIO: Genetics of the Daleks)

The Tenth Doctor introduced himself as the Lord of Time to Madame de Pompadour's lover, King of France Louis XV. (TV: The Girl in the Fireplace)

Claiming the title
During his visit to Bowie Base One in 2059, the Tenth Doctor came to believe that, as the last of the Time Lords, he should be in control over the Laws of Time. He demonstrated this by breaking a fixed point in time to rescue Adelaide Brooke, Mia Bennett and Yuri Kerenski from the Flood. In a subsequent conversation with Adelaide, he confirmed her suggestions that he was effectively omnipotent and unstoppable; she decried the implications of his actions as "wrong" due to how "nobody should have [such] power".

Following Adelaide's subsequent suicide, the Tenth Doctor saw a vision of Ood Sigma and returned to the TARDIS, pondering the omens of his regeneration. (TV: The Waters of Mars) The change to history created a Time Fracture (PROSE: What the TARDIS thought of "Time Lord Victorious") which the Doctor used to flee to the Dark Times. There, he seized the opportunity to end death by infecting the Kotturuh with a modified version of their own infection, believing he could completely rewrite history for the better. After unleashing it on the Kotturuh on Andalia, he again declared himself the "Time Lord Victorious", claiming that his word was eternal.

Dismissing his internal concerns that he wasn't doing the right thing, the Doctor led the Victis Fleet to finish the Kotturuh by destroying their planet, Mordeela, and seal off their power source. However, he was interrupted by the Eighth and Ninth Doctors, allied with the Dalek Time Squad and Vampires. (PROSE: The Knight, The Fool and The Dead) The Tenth Doctor refused to back down and ordered his fleet to fire, destroying the planet. This began the Battle of Mordeela, resulting in the destruction of the Tenth Doctor's fleet, except for his flagship. In the damage, he reflected that the Time Lord Victorious had been beaten, but Brian the Ood encouraged him to fight another day. The Doctor managed to use Mordeela's ruins to seal the gateway to the Kotturuh's power source as intended and used that as a distraction to enable his ship to escape. (PROSE: All Flesh is Grass)

Afterwards, the Doctor continued harbour doubts about what he had done, seeking the Minds of Magnox for an answer about whether he was in the right. When the Kotturuh's ship laid waste to Magnox after Brian killed the Minds, thus depriving them of the chance to ask how to defeat the Doctor and survive his death sentence, the Doctor helped a few Magnoxians flee to Islos. He was rebuked by Peschell for having brought the Kotturuh down on them, with her horrified by him offering to let her travel with him. Though solemn, the Doctor insisted to Brian that he had to continue his war as the Time Lord Victorious. (AUDIO: The Minds of Magnox)

The Doctor eventually came to regret his actions after meeting Inyit, the last of the Kotturuh, who revealed the Kotturh were not what they had appeared to be, and he discarded the "Time Lord Victorious" completely. However, he suggested the Eighth Doctor taunt the Daleks with the title when he detonated an explosive aboard their saucer during the Defence of Gallifrey, with the ensuing explosion forcing the Dalek ship out of the Dark Times. (PROSE: All Flesh is Grass)

Other realities
In an alternate timeline envisioned by a Dalek continuity bomb, rather than exposing himself to lethal radiation to rescue Wilfred Mott at Joshua Naismith's mansion, the Tenth Doctor judged himself to be more important to live and let Wilf die. He reclaimed the "Time Lord Victorious" title and became emperor of the universe, demanding tributes from ambassadors from across the universe to help feed the starving. He was ultimately assassinated by a Raxacoricofallapatorian. (COMIC: Four Doctors)

In the Daft Dimension, the Tenth Doctor claiming of the title mostly amounted to him putting on Time Lord robes with an outrageously large collar, which proved a hazard to the universe for purely physical reasons. (COMIC: The Daft Dimension 556)

Legacy
The Testimony Foundation had footage of the Tenth Doctor walking away from Adelaide Brooke after proclaiming himself the Time Lord Victorious, which forced the Twelfth Doctor to remember her death. (PROSE: Twice Upon a Time)

Behind the scenes

 * The alternate timeline where the Doctor becomes emperor of the universe before being assassinated closely resembles the story of the Emperor from PROSE: Father Time, who was revealed in PROSE: The Gallifrey Chronicles to be a future incarnation of the Eighth Doctor.
 * At VividCon 2008, a fan video debuted which juxtaposed shots from series 2, 3, and 4 episodes with the Flobots song "Handlebars", suggesting that the superficially carefree and easy-going Tenth Doctor was descending into increasingly dark and destructive actions. Writer Paul Cornell praised the video's character study as "brilliant and shocking". In homage to the video, the Tenth Doctor's subsequent turn to Time Lord Victorious in TV: The Waters of Mars has been described by some fans and writers as going "full handlebars".