Gostak
Gostak was a Time Lord who taught at the Time Lord Academy, tutoring the First Doctor before leaving Gallifrey to explore as "the Pilgrim". According to the Fifth Doctor, he went to find the lost moon of Botoya. He once landed in Syria and a myth around him and his TARDIS. When Gostak found the moon, he intended to use its powers to rewrite history and create a new Time Lord Empire. Discovering that the device needed four Time Lords to operate it, Gostak split Botoya's chroni-core into three pieces, hoping that each would be found by an incarnation of the Doctor.
The Fifth, Sixth and Eighth Doctors each discovered a piece of the core before being brought together by Vakrass. When the Doctors confronted their old tutor, they refused to aid him before Gostak threatened to harm their companions. Pretending to agree, the Doctors took the opportunity to turn Botoya's energy field back on itself, decaying it into its future form of the dead asteroid Primeline-275. When the Seventh Doctor formed an exit portal, Gostak refused to abandon Botoya, Vakrass staying behind to ensure that the rogue Time Lord died with the moon. (AUDIO: The End of the Beginning)
Incidentally, the Pilgrim was the name taken by a version of the Doctor in another reality. (AUDIO: The Key To Key To Time)
