The War Master

This version of the Master is played by Sir Derek Jacobi. It's almost crazy that Sir Derek would want to return to a role that he only played briefly on TV, and for most of that time, imagined himself to be a different character altogether. However, he apparently saw just how much fun Sir John Hurt had playing the War Doctor, and wanted to take a swing. The apparent continuity snarl is solved thus: The Reborn Master at some point regenerates into the War Master, who goes on to have a lot of adventures during the Time War. He's mostly concurrent with the Eighth Doctor, who spent a lot of time avoiding the war, so they didn't meet (with rare exceptions). Eventually, the Master deduced that the Time Lords were losing the war, so he used a Chameleon Arch to turn himself into a human infant, who then grew up to become Professor Yana, where the Doctor meets him in "Utopia." Got it? Good, because this is a fairly big section.