Sebastian Grayle

Sebastian Grayle was a man who made a deal with the Nimon for immortality. Repeated frustrations by the Eighth Doctor made him obsessed with killing the Time Lord. (BFA: Seasons of Fear)

Biography
In 305 AD, the Romans had an advance fort set up to protect Britannia from the aggressive forces of the Picts. One of the soldiers there was Decurion Sebastius Gralae. Having seen his older brother claim the family inheritance, Gralae had enlisted in the Roman Legion as a means of gaining the wealth and stature he would need to marry his beloved, Julia. When the communicator from a Nimon scout ended up in his possession, he negotiated a deal with them to sacrifice all the men in his fort in exchange for immortality. Grayle would then have the time to build the fortune he would need to take the family inheritance from his brother. The Doctor stepped in to prevent the sacrifice of the fort's men, but couldn't stop a partial release of power--enough to make Gralae impervious to harm, and to keep him alive for centuries.

In 1055, the stars had aligned properly for the Nimon to try again. Grayle, now going by the name "Leofric of Exeter", had prepared for the event by building a transport capsule to the Nimon's design. The Nimon also taught him how and where to mine uranium and turn it into plutonium to power the capsule. Grayle staged an assault on the court of Edward the Confessor, but the Doctor's interference again prevented him from succeeding.

By 1806, the stars were in alignment again, and Grayle was ready. He had replenished his supply of plutonium, and had rebuilt the capsule in Wycombe Caves with the help of the Hellfire Club. Once again, the Doctor appeared and managed to strip away Grayle's immortality, but not before Grayle succeeded in bringing the Nimon to Earth. The Doctor used his TARDIS to block the Nimon's travel, however, and Grayle gave chase in the Nimon capsule for a confrontation within the Time Vortex.

In one turn of events, the Doctor was thrown into the Vortex and perished. Grayle successfully handed the Earth and the TARDIS to the Nimon, who used Earth to take over all of space and time. Unsatisfied with his revenge on the Doctor, Grayle asked the Nimon to create a variant timeline where he could meet up with the Doctor in Singapore and gloat about his victory. The meeting, quite satisfying for Grayle, initially set the Doctor on his travels to stop Grayle's actions.

In another turn of events, the Doctor traveled through the Time Vortex back to 4th century Britannia, at the Roman fort. There, the Doctor gave the original Grayle the riches he would need to marry Julia, preventing the entire course of events. The future Grayle was furious with this last attempt to stop his ascension to power. The past Gralae, horrified by the example of what he might become, killed the future Grayle and then thanked the Doctor for his kindness. (BFA: Seasons of Fear)

Behaviour
Grayle was initially a pious man, with a firm belief in Mithras. However, after his contact with the Nimon, he lost his belief in gods, and believed only in himself.

As he progressed past a thousand years of age, Grayle became more distant from people and much more coarse in attitude. He married over a dozen times, only to see all of his wives grow old and die while he remained untouched by time's hand. Eventually, he lost all caring for people, concerned only with attaining the godlike powers he had been promised.