The Valeyard

"There is some evil in all of us, Doctor, even you. The Valeyard is an amalgamation of the darker sides of your nature, somewhere between your twelfth and final incarnation."

- The Master about the Valeyard

The Valeyard is the main antagonist of the 14-part Doctor Who serial "The Trial of a Time Lord". He is a lawyer and a future incarnation of the Doctor himself.

History
As the Valeyard doesn't have much regenerations left, he wanted to get the Doctor accused in a trial for death penalty. However, the Master appeared in the trial to point out that the Valeyard was actually the Doctor himself, which made him flee the trial into the Matrix, where the Doctor followed him. In the end the Doctor could escape the death sentence, but also the Valeyard was able to escape.