Darius I (Old Persian Dârayavauš): king of ancient Persia, whose reign lasted from 522 to 486. He seized power after killing king Gaumâta, fought a civil war (described in the Behistun inscription), and was finally able to refound the Achaemenid empire, which had been very loosely organized until then. Darius fought several foreign wars, which brought him to India and Thrace. When he died, the Persian empire had reached its largest extent. He was succeeded by his son Xerxes.
In the Behistun inscription, Darius tells about his coup d'état and the civil war. The full text of the inscription can be found here.
(11) King Darius says: Afterwards, there was a certain man, a Magian, Gaumâta by name, who raised a rebellion in Paishiyâuvâdâ, in a mountain called Arakadriš. On the fourteenth day of the month Viyaxana [11 March 522] did he rebel. He lied to the people, saying: 'I am Smerdis, the son of Cyrus, the brother of Cambyses.' Then were all the people in revolt, and from Cambyses they went over unto him, both Persia and Media, and the other provinces. He seized the kingdom; on the ninth day of the month Garmapada [1 July 522] he seized the kingdom. Afterwards, Cambyses died of natural causes.
(12) King Darius says: The kingdom of which Gaumâta, the Magian, dispossessed Cambyses, had always belonged to our dynasty. After that Gaumâta, the Magian, had dispossessed Cambyses of Persia and Media, and of the other provinces, he did according to his will. He became king.(13) King Darius says: There was no man, either Persian or Mede or of our own dynasty, who took the kingdom from Gaumâta, the Magian. The people feared him exceedingly, for he slew many who had known the real Smerdis. For this reason did he slay them, 'that they may not know that I am not Smerdis, the son of Cyrus.' There was none who dared to act against Gaumâta, the Magian, until I came. Then I prayed to Ahuramazda; Ahuramazda brought me help. On the tenth day of the month Bâgayâdiš [29 September 522] I, with a few men, slew that Gaumâta, the Magian, and the chief men who were his followers. At the stronghold called Sikayauvatiš, in the district called Nisaia in Media, I slew him; I dispossessed him of the kingdom. By the grace of Ahuramazda I became king; Ahuramazda granted me the kingdom.
(14) King Darius says: The kingdom that had been wrested from our line I brought back and I reestablished it on its foundation. The temples which Gaumâta, the Magian, had destroyed, I restored to the people, and the pasture lands, and the herds and the dwelling places, and the houses which Gaumâta, the Magian, had taken away. I settled the people in their place, the people of Persia, and Media, and the other provinces. I restored that which had been taken away, as is was in the days of old. This did I by the grace of Ahuramazda, I labored until I had established our dynasty in its place, as in the days of old; I labored, by the grace of Ahuramazda, so that Gaumâta, the Magian, did not dispossess our house.
(15) King Darius says: This was what I did after I became king.