Fayum was a priest in service to the Egyptian royal court at around the time of Erimem's planned coronation. He was desirous of his queen's ascension, and did what he could to squash rumours about other potential heirs to Erimem's father.
Unlike his ally, Antranak, he was less suspicious of the newly-arrived Fifth Doctor and Peri. He even tended the Doctor medically while the Time Lord suffered from snake poison. Because he believed the poison was universally terminal, he was amazed when the Doctor suddenly arose, as if he had merely been sleeping. Because he kept close watch over the poisoned time traveller, Fayum was one of only a few people in history to determine that the Doctor had a binary vascular system.
He then continued to stick by the Doctor's side, since this was what he had promised his queen. When the Doctor went to talk with Yanis at the mercenary camp, therefore, Fayum was along for the ride. There, he was shocked to discover that he was the object of what he considered to be the seditious plans of Horemshep to instal another person on the throne of Egypt. The older priest contended that it was Fayum himself who belonged on the throne of Egypt, because he was the alleged illegitimate half-brother of Erimem through a woman named Varela.
By now, though, Fayum had been infected by the stasis box entity and his mind was more amenable to Horemshep's story. Although the Doctor managed to discredit the older priest's story, Fayum was still in the grip of the entity's mind control, and willing to try alternate avenues to power. Briefly, he was completely under Horemshep's guidance, and in this state of mind took Peri hostage. This physical contact allowed the entity to infect Peri as well.
Soon, however, the Doctor and Erimem were able to render the entity harmless by trapping it in the body of a cat, and Fayum returned to normal. Now in his right mind, he challenged Yanis to a duel and killed him.
Though she[who?] did not believe Horemshep's claim that he had been controlled by the entity, she did fully believe and pardon Fayum. Fayum then went on to witness Erimem's "miraculous" departure in the TARDIS — and, Erimem guessed, became pharaoh in her stead, even though he was not truly her father's son. (AUDIO: The Eye of the Scorpion)