An Immortal, Darius' first teacher is a mysterious figure, little is actually known of him.
His homeland, Parthia, was part of the old Persian Empire, located in what is now northeastern Iran. The name Ahasuerus appears in the Bible, but does not necessarily refer to a famous individual, however, in a legendary and historical context, Ahasuerus was the Persian king who made Esther the Israelite his queen, and historians have equated him with Xerxes I, heir of Darius the Great of the Achaemenid Dynasty of Persia. Xerxes and his heir (also named Darius) were both murdered in 465 B.C. as a result of a plot by Artabanus, the commander of the palace guard.
While we know nothing of their student-teacher relationship, he must have taught Darius the Rules and the skills that would enabled him to survive in the Game, so in that respect he could be considered a friend. The Greek historian, Herodotus, portrayed Xerxes I as headstrong, temperamental, governed by passion, and prone to executing people (even his relatives) on a whim. If he was indeed Darius' first teacher he may have passed some of his autocratic attitudes on to his student.
Note: He appeared only as an entry in the Watcher Chronicles CD.