Darius I in Wikipedia
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.
Darius left a tri-lingual monumental relief on Mount Behistun which was written in Elamite, Old Persian and Babylonian between his coronation and his death. The inscription first gives a brief autobiography of Darius with his ancestry and lineage. To expand on his ancestry, Darius left a sequence of events that occurred after the death of Cyrus the Great. Darius mentions several times that he is the rightful emperor by the grace of Ahura Mazda, the Zoroastrian God. In addition, further texts and monuments from Persepolis have been found, including a fragmentary Old Iranian inscription from Gherla, Rumania (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 emperors and the Greco-Persian Wars. He wrote an extensive amount of information on Darius which spans half of book 3, along with books 4, 5 and 6; it begins with removal of the alleged usurper Gaumata and continues to the end of Darius's reign. 
The Book of Ezra (chapter 6, verse 1) describes the adoption and precise instructions to rebuild the Temple in Jerusalem. It was completed and inaugurated of the sixth year of Darius (March 515 BCE), as also related in the Book of Ezra (chapter 6, verse 15), so the 70-year prophecy of Jeremiah was fulfilled. Between Cyrus and Darius, an exchange of letters with King Ahasuerus and Artaxerxes is described (Chapter 4, Verse 7), the grandson of Darius I, in whose reign Ezra and Nehemiah came to Jerusalem. The generous funding of the temple gave Darius and his successors the support of the Jewish priesthood. There is mention of a Darius in the Book of Daniel, identified as Darius the Mede. He began ruling when he was 62 years old (chapter 5, verse 31), appointed 120 satraps to govern over their provinces or districts (chapter 6, verse 1), was made king over the realm of the Chaldeans (chapter 9, verse 1), and predated Cyrus (chapter 11, verse 1). Therefore, many scholars identify him with Cyaxares II rather than Darius I of Persia.
Birth and youth
Darius was born as the eldest son to Hystaspes and Rhodugune in 550 BCE. Hystaspes was a leading figure of authority in Persis which was the homeland of the Persians. Darius' 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 emperor.
Rise to power
The rise of Darius to the throne contains two different sides to the story. Our sources (the Bisitun inscription and Herodotus) both give similar stories (below). However, historians have inferred from these that Darius' rise to power may have been illegitimate. 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. The fact that Darius' father and grandfather are still alive (Bisitun Inscription) implies that he was not the next in line to a hereditary throne.
The account of Darius which is 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' 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, he along with Otanes, Intraphrenes, Gobryas, Hydarnes, Megabyxus and Aspathines killed Gaumata in the fortress of Sikayauvati. Darius was proclaimed emperor.
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 emperor. 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 emperor the following morning.
After Bardiya was murdered, widespread revolts occurred throughout the empire, especially on the eastern side. Darius asserted his position as emperor by force, taking his armies throughout the empire, suppressing each revolt individually. The most notable of all the revolts is the Babylonian revolt which was led by Nebuchadnezzar III. This revolt occurred when Otanes had taken a large amount of the army out of Babylon to aid Darius in suppressing other revolts. Darius felt that the Babylonian had taken advantage of him and deceived him, which resulted in Darius gathering up a large army and marching to Babylon. At Babylon, Darius was met with closed gates and a series of defenses to keep him and his armies out of Babylon. 
Darius encountered mockery and taunting from the rebels, including the famous saying "Oh yes, you will capture our city, when mules shall have foals." For a year and a half, Darius and his armies were unable to capture Babylon, though he attempted many tricks and strategies -- even copying that which Cyrus the Great had employed when he captured Babylon. However, the situation changed in Darius's favor when one of Zopyrus's mules foaled (at the time, people believed such a birth to be a great miracle, the product of a divine act). Following this, a plan was created for a high-ranking soldier to pretend to be a deserter, enter the Babylonian camp, and gain the trust of the Babylonians. The plan was successful, and the Persians eventually surrounded the city and conquered the rebels.
During this revolt, Scythian nomads took advantage of the disorder and chaos and invaded southern Persia. Darius first finished defeating the rebels in Elam, Assyria, and Babylon, then attacked the Scythian invaders. He pursued the invaders, who led him to a marsh; there he found no known enemies but an enigmatic Scythian tribe distinguished by their large pointed hats. Even though Darius's campaign against the Scythians largely ended in failure, he soon would achieve great successes in his European campaigns.
Persian invasion of Greece
Main article: First Persian invasion of Greece
Darius's European expedition was a major event in his reign. Starting with the Scythians, Darius conquered Scythia, Thrace and many cities of the northern Aegean, while Macedonia submitted voluntarily. The Asiatic Greeks and Greek islands had submitted to Persian rule by 510 BCE. They were being governed by tyrants responsible to Darius.
Nonetheless, there were certain Greeks who were pro-Persian, such as the Medizing Greeks, which were largely grouped at Athens. This improved Greek-Persian relations as Darius opened his court and treasuries to the Greeks who wanted to serve him. These Greeks served as soldiers, artisans, statesmen and mariners for Darius. However, Greek fear of the Persians becoming very strong and the constant interference by the Greeks in Ionia and Lydia were all stepping stones in the conflict that was yet to come between Persia and Greece.
When Aristagoras organized the Ionian revolt, Eretria and Athens supported him by sending ships to Ionia and burning Sardis. Persian military and naval operations to quell the revolt ended in the Persian reoccupation of Ionian and Greek islands. However, anti-Persian parties gained more power in Athens, and pro-Persian aristocrats were exiled from Athens and Sparta. Darius responded by sending a group of troops led by his son-in-law across the Hellespont. However, the battle was lost due to a violent storm and harassment by Thracians.
Determined to punish Athens, Darius sent a second army consisting of 20,000 men under Datis who captured Eretria and moved onwards to Marathon. In 490, at the Battle of Marathon, the Persians were defeated by a heavily armed Athenian army with 9,000 men who were supported by 600 Plataeans and 10,000 lightly armed soldiers led by Miltiades. The defeat at Marathon marked the end of the First Persian invasion of Greece.
After becoming aware of the Persian defeat at the Battle of Marathon, Darius began planning another expedition against the Greek-city states; this time, he, not Datis, would command the imperial armies. 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 BCE, the body of Darius was embalmed and entombed in the rock-cut sepulcher which had been prepared for him several years earlier.
Xerxes, eldest son of Darius and Atossa, succeeded to the throne as Xerxes I. However, prior to Xerxes's accession, he contested the succession with his elder half-brother Artobazan, Darius' oldest son who was born to his commoner first wife before Darius rose to power. 
The cuneiform inscriptions on Darius's tomb were squeezed and made into negative forms by the German archaeologist Ernst Herzfeld in 1923. They are currently housed in the archives of the Freer Gallery of Art and Arthur M. Sackler Museum, Smithsonian Institution, Washington, DC.
Early in his reign, Darius wanted to organize the loosely organized empire with a system of taxation which had been passed down to him from Cyrus and Cambyses. To do this, Darius created twenty provinces called satrapies (or archi) which were each assigned to a satrap (archon) and specified fixed tributes that the satrapies were required to pay. A complete list is preserved in the catalog of Herodotus, beginning from Ionia and listing the other satrapies from west to east excluding Persis which was the land of the Persians and the only province which was not a conquered land. Tributes were paid in both silver and gold talents. The tributes from each satrap that were paid in silver were measured with the Babylonian talent, and those paid in gold were measured with the Euboic talent. The total tribute from the satraps came to a number less than 15,000 silver talents.
The majority of the satraps were of Persian origin and were members of the royal house or the six great noble families. These satraps were personally picked by Darius to monitor these provinces, which were divided into sub-provinces with their own governors which were chosen either by the royal court or by the satrap. The assessment of the tribute was accomplished by Darius sending a commission of men to evaluate the expenses and revenues of each satrap. To ensure that one person did not gain too much power, each satrap had a secretary who observed the affairs of the state and communicated with Darius, a treasurer who safeguarded provincial revenues, and a garrison commander who was responsible for the troops. Additionally, royal inspectors who were the "eyes and ears" of Darius completed further checks over each satrap.
There were headquarters of imperial administration at Persepolis, Susa, and Babylon while Bactria, Ecbatana, Sardis, Dascyclium and Memphis also had branches of imperial administration. Darius chose Aramaic as a common language, which soon spread throughout the empire. However, Darius gathered a group of scholars to create a separate language system only used for Persis and the Persians, which was called Aryan script which was only used during official inscriptions.
Before 500 BCE, Darius had introduced a new monetary system which was based on silver coins with a weight averaging to be around 8g and gold coins averaging to be 5.40 g. The gold coin was called dārayaka (daric) and was probably named after Darius. The coin was primarily used for trade within the empire and even provided a boost in trade. This coin was created to provide a uniform coin and monetary system that would be used within the empire, removing the need for foreign monetary systems. In an effort to further improve trade, Darius built canals, underground waterways, and a powerful navy. He further improved and expanded the network of roads and way stations throughout the empire, so that there was a system of travel authorization by King, satrap, or other high official, which entitled the traveler to draw provisions at daily stopping places. 
Darius was an adherent of Zoroastrianism and believed that Ahura Mazda had appointed him to rule the Persian Empire. At inscriptions, such as the Behistun Inscription, he mentions that he believes he is chosen by Ahura Mazda to be the emperor. Darius had dualistic convictions and believed that each rebellion in his kingdom was the work of Drug, the enemy of Asha. Darius believed that because he lived righteously by Asha, Ahura Mazda supported him.  In many cuneiform inscriptions denoting his achievements, he presents himself a devout believer perhaps even convinced that he had a divine right to rule over the world. 
In the lands that were conquered by the Persian Empire, Darius followed the same Achaemenid tolerance that Cyrus had shown, and later Achaemenid emperors would show. He supported faiths and religions that were "alien" as long as the adherents were submissive and peaceable, sometimes giving them grants from his treasury for their purposes.  He had funded the restoration of the Jewish temple which had originally been decreed by Cyrus the Great, presented favour towards Greek cults which can be seen in his letter to Gadatas, and supported Elamite priests. He had also observed Egyptian religious rites related to kingship and had built the temple for the Egyptian God, Amun.
During Darius's Greek expedition, he had begun construction projects in Susa, Egypt and Persepolis. He had linked the Red Sea to the river Nile by building a canal which ran from modern Zaqāzīq to modern Suez. To open this canal, he traveled to Egypt in 497 BCE, where the inauguration was done among great fanfare and celebration. Darius also built a canal to connect the Red Sea and Mediterranean.  On this visit to Egypt, he erected monuments and executed Aryandes on the accounts of treason. When Darius returned back to Persis, he found that the codification of Egyptian law had been finished.
Additionally, Darius sponsored large construction projects in Susa, Babylon, Egypt, and Persepolis. In Susa, Darius built a new palace complex in the north of the city. An inscription states that the palace was destroyed during the reign of Artaxerxes I, but was rebuilt. Today, only glazed bricks of the palace remain with the majority of them in Louvre. In Pasargadae, Darius finished all incomplete construction projects from the reign of Cyrus the Great. A palace was also built during the reign of Darius, with an inscription in the name of Cyrus the Great. It was previously believed that Cyrus had constructed this building, however due to the cuneiform script being used, the palace is believed to have been constructed by Darius. In Egypt, Darius built many temples, and restored those that had previously been destroyed. Even though Darius was a Zoroastrian, he built temples dedicated to the Gods of the Ancient Egyptian religion. Several temples found were dedicated to Ptah and Nekhbet. Darius also created several roads and routes in Egypt. The monuments that Darius built were often inscribed in the official languages of the Persian Empire, which were Old Persian, Elamite, Babylonian and Egyptian hieroglyphs. To construct these monuments, Darius had hired a large number of workers and artisans of diverse nationalities. Several of these workers were deportees who had been employed specifically for these projects. These deportees enhanced the economy and improved international relations with neighboring countries that these deportees arrived from. During the period of Darius's death, construction projects were still underway. Xerxes completed these works and in some cases expanded his father's projects by erecting new buildings of his own.