Darius ascended the throne by overthrowing Gaumata, a usurper. The new king met with rebellions throughout his kingdom and quelled them each time. A major event in Darius's life was his expedition to punish Athens and Eretria for their aid in the Ionian Revolt and subjugate Greece. Although ultimately ending in failure at the Battle of Marathon, Darius succeeded in the re-subjugation of Thrace, expansion of the empire through the conquest of Macedon, the Cyclades and the island of Naxos and the sacking of the city of Eretria.
Darius organized the empire by dividing it into provinces and placing satraps to govern it. He organized a new uniform monetary system, along with making Aramaic the official language of the empire. He also put the empire in better standing by building roads and introducing standard weights and measures. Through these changes, the empire was centralized and unified. Darius also worked on construction projects throughout the empire, focusing on Susa, Pasargadae, Persepolis, Babylon and Egypt. He had the cliff-face Behistun Inscription carved to record his conquests, an important testimony of the Old Persian language.
Dārīus and Dārēus are the Latin forms of the Greek Dareîos (Δαρεῖος), itself from Old Persian Dārayava(h)uš (𐎭𐎠𐎼𐎹𐎢𐏁; in Aramaic dryhwš), which is a shortened form of Dārayava(h)uš (𐎭𐎠𐎼𐎹𐎺𐎢𐏁). The longer form is also seen to have been reflected in the Elamite Da-ri-(y)a-ma-u-iš, Babylonian Da-(a-)ri-ia-(a-)muš, Aramaic drywhwš, and possibly the longer Greek form Dareiaîos (Δαρειαῖος). The name is a nominative form meaning "he who holds firm the good(ness)", which can be seen by the first part dāraya, meaning "holder", and the adverb vau, meaning "goodness".
At some time between his coronation and his death, Darius left a tri-lingual monumental relief on Mount Behistun, which was written in Elamite, Old Persian and Babylonian. The inscription begins with a brief autobiography including his ancestry and lineage. To aid the presentation of his ancestry, Darius wrote down the sequence of events that occurred after the death of Cyrus the Great. Darius mentions several times that he is the rightful king by the grace of Ahura Mazda, the Zoroastrian god. In addition, further texts and monuments from Persepolis have been found, as well as a clay tablet containing an Old Persian cuneiform of Darius from Gherla, Romania (Harmatta) and a letter from Darius to Gadates, preserved in a Greek text of the Roman period.
Herodotus, a Greek historian and author of The Histories, provided an account of many Persian kings and the Greco-Persian Wars. He wrote extensively on Darius, spanning half of Book 3 along with Books 4, 5 and 6. It begins with the removal of the alleged usurper Gaumata and continues to the end of Darius's reign.
Darius was the eldest of five sons to Hystaspes and Rhodugune in 550 BCE. Hystaspes was a leading figure of authority in Persia, which was the homeland of the Persians. Darius's inscription states that his father was satrap of Bactria in 522 BCE. According to Herodotus, Hystaspes was the satrap of Persis, although most historians state that this is an error. Also according to Herodotus (III.139), Darius, prior to seizing power and "of no consequence at the time", had served as a spearman (doryphoros) in the Egyptian campaign (528–525 BCE) of Cambyses II, then the Persian Great King; this is often interpreted to mean he was the king's personal spear-carrier, an important role. Hystaspes was an officer in Cyrus' army and a noble of his court.