Twelfth Doctor

This alternate version of the Twelfth Doctor came into existence due to a continuity bomb after being selected by his prime self due to them believing his alternate reality was the safest. However, plagued by loneliness and regret, this version of the Doctor was cold, calculating and bent on replacing the Time Lords he had spent years searching for as the ruler of the universe. (COMIC: Four Doctors)

History
After he sent his companion Clara Oswald away due to her betrayal, the Doctor spent centuries searching the universe for Gallifrey, until he gave up in despair and found the city of the Voord. After years among them, they offered him leadership of the group mind. The Doctor accepted and, upon joining them, began to remember the continuity bomb and how he was responsible for the ordeal surrounding it.

Leaving the clues that got him there, the alternate Doctor waited on Marinus for his past self, along with his tenth incarnation and eleventh incarnations and their companions, and then fired upon them to direct them towards the continuity bomb.

After they arrived in his timeline, the alternate Doctor took them to the Voord city, where he revealed himself as both the Voord leader and the mastermind behind the continuity bomb.

Immediately taking the TARDIS and handcuffing the six, the alternate Doctor explained how the Voord hid from the Siege of Trenzalore, and how he had maneuvered the events with the bomb into motion. He explained his plans to wipe their memories and lead to Voord to conquer the universe and become the new Time Lords. However, the three Doctors instigated a struggle that allowed Alice Obiefune and Gabby Gonzalez to escape. The alternate Doctor then took Clara and the three Doctors to a device called the Conscience, to have their memories altered.

As he gloated about his victory, Alice was killed by his Voord guards and Gabby began to frantically open the Doctor's comics in the hope they could help her. Much to her horror, inside the bundle was a miniature Weeping Angel, which grew to full size and sent her back in her own personal timeline to the café before the three Doctors arrived to greet their companions. Gabby warned the companions of what would occur if things went they way they had before, before fading from existence. (COMIC: Four Doctors)

A new timeline
Now prepared for his alternate self's deceptions, the original Twelfth Doctor led his past incarnations and their companions to Marinus to deliberately set of the continuity bomb and enter the Voord city through River Song's World. Once there, the Twelfth Doctor entered the Voord group mind, and was confronted by the alternate Doctor.

The alternate Doctor was able to overpower his younger self, until the Twelfth Doctor called in the Tenth and Eleventh Doctors to assist him. Learning that the Doctors had sabotaged the city's force field, the alternate Doctor surrendered and was erased from existence after Clara apologised for her future betrayal. (COMIC: Four Doctors)

Personality
This version was consumed by guilt at the banishment of his companion Clara so much that he became obsessed with finding the Time Lords, becoming uncaring for the rest of the universe in his isolation. His original self saw through his ruse of cooperation, realising he had descended into a madness to rule the universe and replace their original self. He was also somewhat arrogant, boasting that his plan could not be stopped by his younger selves. (COMIC: Four Doctors)

Appearance
Compared to his prime self, this version of the Twelfth Doctor was much more ragged, being much older and sporting a white beard with overgrown white hair to match. He had also changed his attire as well, no longer sporting an overcoat. (COMIC: Four Doctors)