Accession
The rise of Darius to the throne contains two variations, an account from Darius and another other from Greek historians. Some modern historians have inferred that Darius's rise to power might have been illegitimate. To them, it seems likely that Gaumata was in fact Bardiya, and that under cover of revolts, Darius killed the heir to the throne and took it himself.
Darius's account, written at the Behistun Inscription states that Cambyses II killed his own brother Bardiya, but that this murder was not known among the Iranian people. A would-be usurper named Gaumata came and lied to the people, stating he was Bardiya. The Iranians had grown rebellious against Cambyses's rule and on 11 March 522 BCE a revolt against Cambyses broke out in his absence. On 1 July, the Iranian people chose to be under the leadership of Gaumata, as "Bardiya". No member of the Achamenid family would rise against Gaumata for the safety of their own life. Darius, who had served Cambyses as his lance-bearer until the deposed ruler's death, prayed for aid and in September 522 BCE, along with Otanes, Intraphrenes, Gobryas, Hydarnes, Megabyxus and Aspathines, killed Gaumata in the fortress of Sikayauvati.
Several days after Gaumata had been assassinated, Darius and the other seven nobles discussed the fate of the empire. At first, the seven discussed the form of government; a democratic republic was strongly pushed by Otanes, a oligarchy was pushed by Megazybus, while Darius pushed for a monarchy. After stating that a republic would lead to corruption and internal fighting, while a monarchy would be led with a single-mindedness, not possible in other governments, Darius was able to convince the other nobles that a monarchy was the correct form of government. To decide who would become the monarch, the six nobles (Otanes stated that he had no interest in becoming king) decided on a test. All six nobles would gather outside mounted on their horses at sunrise, and the nobles' horse which neighed first would become Great King. According to Herodotus, Darius had a slave, Oebares who helped Darius win this contest. Before the contest, Oebares rubbed his hand over the genitals of a mare that Darius's horse had a fondness for. When the six nobles gathered outside, Oebares placed his hands beside the nostrils of Darius's horse, who became excited at the smell and neighed. Immediately after, lightning and thunder occurred leading the other six noblemen to believe to be an act of God, causing them to dismount and kneel before Darius. Darius did not believe that he had achieved the throne through fraud but through brilliant sagacity, even erecting a statue of himself mounted on his neighing horse stating "Darius, son of Hystaspes, obtained the sovereignty of Persia by the sagacity of his horse and the ingenious contrivance of Oebases, his groom."
According to the accounts of Greek historians, Cambyses II had left Patizeithes in charge of the kingdom when he headed for Egypt. He later sent Prexaspes to murder Bardiya. After the killing, Patizeithes put his brother Gaumata, a Magian who resembled Bardiya, on the throne and declared him the Great King. Otanes discovered that Gaumata was an impostor, and along with six other Iranian nobles including Darius, created a plan to oust the pseudo-Bardiya. After killing the impostor along with his brother Patizeithes and other Magians, Darius was crowned king the following morning.
Read more about this topic: Darius I