Darius I, known as Darius the Great, was the third "king of kings" (emperor) of the Achaemenid Empire. Darius held the empire at its peak, then including Egypt, northern India, and parts of Greece. The decay and downfall of the empire commenced with his death and the coronation of his son, Xerxes I.
Darius ascended the throne by assassinating the alleged usurper Bardiya with the assistance of six other Persian noble families; Darius was crowned the following morning. The new emperor 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. Darius expanded his empire by conquering Thrace and Macedon, and invading the Saka, Iranian tribes who had invaded Medes and had previously killed Cyrus the Great.
Darius organized the empire, by dividing it into provinces and placing governors to govern it. He organized a new uniform monetary system, along with making Aramaic the official language of the empire. Darius also worked on construction projects throughout the empire, focusing on Susa, Pasargadae, Persepolis, Babylon, and Egypt. Darius created a codification of laws for Egypt. He also carved the cliff-face Behistun Inscription, an autobiography of great modern linguistic significance. After becoming aware of the Persian defeat at the Battle of Marathon, Darius began planning another expedition against the Greek-city states. Darius had spent three years preparing men and ships for war when a revolt broke out in Egypt. This revolt in Egypt worsened his failing health and prevented the possibility of leading another army himself; soon, Darius was dead. In October 486 BC, the body of Darius was embalmed and entombed in the rock-cut sepulcher which had been prepared for him several years earlier.