First Master

The eleventh incarnation of The Master, but the first to go by "The Master", this incarnation regenerated from the tenth incarnation of Koschei, who had been killed by Astrel, like many of his other incarnations. In this incarnation, Koschei, rejected his name, calling himself The Master. He became evil at Theta Sigma, for betraying him, and wanted to kill him. This is when The Master first became enemies with Theta Sigma, who would be The Doctor.

Bio
The tenth incarnation of Koschei was a tiny, ancient creature (similar to the Tenth Doctor after being aged a lot by The Master). After being mortally wounded by Astrel, the tenth incarnation shined with a golden light, and became his eleventh incarnation. His eleventh incarnation was an albino with long hair. Tired of suffering, Koschei decided he hated Theta Sigma, and that he betrayed him, even though they'd been good friends in the past. He finally gave his location to Astrel. Pleased, Astrel let him go, releasing him on Gallifrey.

Suffering from post regeneration trauma, Koschei was found by his son, Cadmus, and wife, Alexandra. They took care of him until he recovered and got used to his new body.

Theta Sigma was abducted and tortured by Astrel. This pleased Koschei. Eventually, Theta Sigma had regenerated many times, he had lost 7 regenerations (he had regenerated 8 times since he had regenerated before, whenever Astrel first attacked). He had enough regeneration energy to cause an explosion, damaging Astrel's hideout. Newly regenerated, and in his ninth incarnation, Theta Sigma had a sword fight with Astrel, and killed him. Astrel's body fell into the vacuum of space and burned up. Using Astrel's gauntlet, Theta Sigma defeated Xasergast.

After this, Theta Sigma was given a new regeneraton cycle, because of how many regenerations he had lost. He was awarded for defeating Xasergast and Astrel. In his new incarnation, which became known as the "First Doctor", later on, Theta Sigma watched his granddaughter, Susan, grow up.

Cadmus and Alexandra had been killed by a meteor, and Koschei rejected his name and was no longer governor, becoming a wanderer and terrorist. He went by "The Master", and plotted against The Doctor. Everything in his life was gone.