The War King
The War King was an incarnation of the Master destined to become the leader of the Time Lords during the Second War in Heaven.
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During the events of the Second War in Heaven, the Master was captured and interrogated about his world by the agents of the Enemy. The Master managed to escape, but the torture deeply affected the Master, it made him realise how he cared for his people. The Master concluded that he would help the Time Lords in any way he could. The gossips said that he just took advantage of this opportunity to gain the immunity for his past crimes and the power over Gallifrey.
The Master arrived on Gallifrey and told the Time Lords of what he had witnessed and vowed to help the Time Lords in any way they wished. President Romana refused to accept the Enemy's existence and their threat to Gallifrey and she warned the Master that if he ever returned to Gallifrey to unsettle Time Lord society with speak of a War; he would be arrested for his previous crimes against the Time Lords.
The Master's actions on Gallifrey had caused some Time Lords to investigate the existence of the Enemy and their threat to Gallifrey, to which the Time Lords discovered that the Enemy was a credible threat and had the power to destroy the Time Lords. Some Time Lords took this further and suggested that the Master be pardoned of his crimes and become part of a future war effort.
The Master saw this banishment as a mere setback and began to devise ways for the Time Lords to combat the Enemy, the Master then spent over 40 years creating plans for new weapons and devices. With the plans he had created, the Master travelled back to Gallifrey in his TARDIS and was captured by the Chancellery Guard, where he denied none of his offences. He then asked for an emergency meeting of the High Council to propose his plans. During the emergency session the Time Lords accpeted that the Master had told the truth and concluded that Gallifrey would need a new leader with military experience. The Master was the obvious candidate and the Time Lords wiped all records of the Master's past and made him War King of the Nine Gallifreys. (FP: The Book of the War)
The War King led the original Gallifrey, also called The Homeworld,(FP: The Book of the War) while Romana ruled Gallifrey VIII, another clone of Gallifrey (EDA : The Ancestor Cell), which believed itself to be THE original Gallifrey. Or perhaps, Romana was true, and the War King was wrong. The War King instigated a policy of alliances with the alien races, and accepted even that the Time Lords and the alien soldiers of Gallifrey can make children together, and many other heresies. But it was the War.
While in power (after 50 years of War) the Master hired House Lolita to destroy the Faction Paradox powerbase of the Eleven-Day Empire to stop the Faction from supplying weapons to the Enemy or sign a treaty of reconciliation, alongside with House Tracolix (and with the help of the Sontarans) with the Faction.
But then, he understood the threat of Lolita, an intelligent and humanoïd TARDIS, who was planning to reach the power on Gallifrey, and contaminate the entire Universe with her own biodata, so she could become a new kind of history (and rewrite the current history of course). The Master even allied himself to the last scions of the Faction Paradox, but was eaten by Lolita, who claimed to herself the title of War Queen. However, Cousine Justine and her army of Mal'akh attacked Lolita directly and took over Gallifrey, whereas Lolita was stuck by the Osirians in a pyramid. So, the Master had won anyway. (FP : Words from Nine Divinities)
(BBV: The Eleven Day Empire)
Behind the scenes
Probably, the Master who became the War King is the same Master who survived the fall in the Eye of Harmony, fought the 8th Doctor, Izzy Sinclair and Kroton (the Glorious Dead), and visited also the 8th Doctor during the babewyns' attack on Earth (EDA : The Adventuress of Henrietta Street). And then, he was probably captured by the agents of the Enemy, only to reach the Homeworld.
It is implied that Gallifrey VIII, destroyed during the events of The Ancestor Cell, was recreated after (EDA : The Gallifrey Chronicles ), only to find itself destroyed again during the Last Great Time War (DW : The End of Time). During the Last Great Time War -which is not the Second War in Heaven-, the Master was resurrected, and in the events of the new series, he didn't displayed any memory of the War King. One explanation is that the Time Lords of Gallifrey VIII believed the Master killed when he fall in the Eye of Harmony, and recreated him with his biodata, from this point. So, two Masters would have been existed.
Or even three Masters, because Father Kreiner, during EDA : Interference, showed the head of two renegades Time Lords hunted down and killed by him, and one of them was just a clone. The Master and the Rani. So, the Master was possibly cloned because he was the War King.
