Decurion Sebastius Gralae, also known as Sebastian Grayle and "Leofric of Exeter", 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.
In 305 AD, the Romans had an advance fort set up to protect Roman Britain from the aggressive forces of the Picts. Amongst the soldiers was Decurion Sebastius Gralae. When his older brother claimed 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 to sacrifice all the men in his fort in exchange for immortality. This would give him time to take the family inheritance from his brother. The Eighth 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.
Grayle found himself a changed man, with fears and doubts. He spent the next 80 years seeking forgiveness with a community of monks in the northern British Isles, watching them age and die while he was untouched by time's hand. He married twelve women, only to watch every one of them fade into old crones. As the centuries passed, the years weighed more heavily on him... and he blamed the Doctor for his misfortunes.
In 1055, the stars 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 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 Eighth 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, the Doctor travelled 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 what he might become, killed the future Grayle and then thanked the Doctor for his kindness. (AUDIO: Seasons of Fear)
As he progressed past a thousand years of age, Grayle became more distant from people and much more coarse in attitude. Eventually, he lost all concern for people, intent on attaining the godlike powers he had been promised. He also developed a near-obsessive hatred of the Doctor, blindly resenting how the Doctor continually interfered with his plans, incapable of acknowledging the Doctor's protests that he would leave Grayle alone if Grayle would abandon his plans for global conquest. (AUDIO: Seasons of Fear)