Amestris was the daughter of Otanes, one of the seven conspirators who killed the Persian rebel king Gaumâta (22 September 522 BCE). After this, Darius I the Great started his reign. According to the Greek researcher Herodotus of Halicarnassus (fifth century), Otanes was honored with a diplomatic marriage: the new king married Otanes' daughter Phaedymia, and Otanes married a sister of Darius, who gave birth to Amestris.
When Darius died in 486, Amestris must have been in her thirties. She was married to the crown prince, Xerxes. Herodotus describes queen Amestris as a cruel despot:
I am informed that Amestris, the wife of Xerxes, when she had grown old, made return for her own life to the god who is said to be beneath the earth by burying twice seven children of Persians who were men of renown.note[Herodotus, Histories, 7.114.]
It is not clear what lies behind this strange story. To the best of our knowledge of the Persian religion, human sacrifices were not permitted. On the other hand, the subterranean god may be identical to Angra Mainyu, "the hostile spirit", who was the eternal enemy of the supreme god Ahuramazda.
According to an oriental fairy tale reproduced by Herodotus, Amestris was a very jealous woman. When Xerxes returned from his Greek war, he fell in love with Artaynte, the wife of one of his sons. In return for her favors, she demanded a special cloak that Amestris had made for Xerxes. When the queen saw her daughter-in-law parading in the royal dress, she knew what was going on, and ordered Artaynte's mother to be mutilated. (Herodotus offers no convincing explanation.) Artayntes' father, Xerxes' brother Masistes, decided to revolt against his king and brother, but was not successful.
This strange story is probably only meant to show that Xerxes was a slave of his women, and therefore unfit to rule. It tells a lot about the Greek ideas about the effeminacy of the Persians, but probably has no historical value.
Amestris remained influential after the death of her husband. During the reign of her son Artaxerxes I (465-424), another son, Achaemenes, was killed by Egyptian rebels. They and their Athenian allies were defeated by the general Megabyzus, who offered terms to the rebels to shorten the war. According to the Greek historian Ctesias of Cnidus (who is not known for his reliability but is our only source), Amestris was enraged because Megabyzus had not punished the murderers of her son. Initially, Artaxerxes did not allow her her revenge, but after five years (in 449?), he permitted her to crucify the Egyptian leader, Inarus, and kill several captives.
Amestris may have died as late as 440.