Darius was one of the great Kings of Persia best known for his administrative genius and building projects. His rise to power was turmoltuous.
According to Herodotus, while Cambyses was in Egypt, a man claiming to be Bardiya (his brother) proclaimed himself king. This man had previously been ordered to death. On Cambyses trip home to sort issues out he died by his own hand. Both Herodotus and the Behistan inscription tell of economic and religious changes made by Bardiya which in turn in accordance with Herodotus put him in favour with the majority of the Persian Empire. No one in the entire Persian Empire realized that Bardiya was an imposter, until Otanes suspected he was not the true son of Cyrus. Otanes was joined by six other conspirators which included Aspathines, Gobryas, Intaphrenes, Megabyzus, Hydarnes, and of course Darius. The conspirator planned the murder of Bardiya and then completed.
The Behistan inscription was written by Darius. Darius defended the murder of Bardiya and his own assumption of kingship. The inscription tells us that it was actually Gaumata, a Magian, who had impersonated Bardiya after Bardiya had been murdered secretly by Cambyses. Darius therefore claimed that he was restoring the kingship to the rightful Achaemenid house. He himself, however, belonged to a collateral branch of the royal family, and, as his father and grandfather were alive at his accession, it is unlikely that he was next in line to the throne. Some modern scholars consider that he invented the story of Gaumata in order to justify his actions and that the murdered king was indeed the son of Cyrus.
Herodotus tells of a story of how Darius became King within the seven conspirators. The first conspirators groom to nay after sunset would become the new King.