Darius was the king of Persia. The Persian Empire around the time of Darius was the greatest power the world had ever known, and Darius was the undisputed king. Yet this mighty king seemed rather gullible when it came to the crafty schemes of a few jealous officials.
They were jealous of the prestige and power of Daniel, one of the Jewish nobles who had been exiled to Babylon many years earlier, because he had been promoted to second-in-command of the Persian Empire. Daniel’s only “flaw” seemed to be his unwavering devotion to God, to whom he prayed three times a day.
So the officials smooth-talked Darius into passing a law requiring everyone to pray only to the king for one month. Daniel, of course, continued to pray to God and was promptly arrested.
When Darius realized that Daniel would be thrown to the lions for his crime, he became distraught, because even he could not revoke anything he had signed into law. So all that was left to do was to pray that God Himself would deliver Daniel.
God did indeed deliver Daniel, and Darius rejoiced and made a new law that everyone should fear the God of Daniel. The Bible mentions a couple of different people named Darius, and both of them are described as ruling over the Persian Empire (Ezra 4:24; Nehemiah 12:22; Daniel 6; Haggai 1:1; Zechariah 1:1). All the references to Darius outside of the book of Daniel probably refer to Darius the Great, who ruled Persia from 522–486 BC.
In the book of Daniel, Darius may be another name for Cyrus the Great, who ruled Persia from 559–529 BC.