Darius, born circa 550 B.C., the son of Hystaspes, was the ruler of the Persian empire from 522 B.C. until he died in 486 B.C..
Having quelled revolts in Mesopotamia, Elam, Iran and Armenia, he went on to conquer the Indus Valley, invade Scythia, and conquered a large chunk of south-east Europe.
The reconstruction of the temple in Jerusalem was completed under his reign in 515 B.C. (cf. Ezra 4-6).
Both Haggai and Zechariah prophesied during his reign.
Shares a name with Darius II (423-404 B.C.); Darius III (336-330 B.C.); Darius the Mede [Daniel 5:31]; and that bloke from Bearsden (described by the Observer as "supremely egotistical") who narrowly missed out on winning ITV's Popstars and Pop Idol competitions.