Who is Sosipatra, and what effect does she have on Julian's life in the novel, Julian?

Sosipatra is a soothsayer or a psychic. When she meets Julian, she predicts that he rises to greatness, power and authority. She foretells that he dies a warrior's death and that he dies in a place called Phrygia. Although this is treasonous talk, because Julian is so superstitious and influenced by signs and omens, he takes this very seriously.