Darius in Fausset's Bible Dictionary

A common name of several Medo-Persian kings, from a Persian root darvesh, "restraint;" Sanskrit, dhari, "firmly holding." 1. Darius the Mede. (See DANIEL; BABYLON; BELSHAZZAR; CYRUS.) Daniel 5:31; Daniel 6:1; Daniel 9:1; Daniel 11:1. This Darius "received the kingdom" (Daniel 5:31) of Babylon as viceroy from Cyrus, according to G. Rawlinson, which may be favored by Daniel 9:1; "Darius, the son of Ahasuerus, of the seed of the Medes, which was made king over the realm of the Chaldaeans." He in this view gave up the kingdom to his superior Cyrus, after holding it from 538 to 536 B.C. Abydenus makes Nebuchadnezzar prophesy that a Persian and a Mede," the pride of the Assyrians," should take Babylon, i.e. a prince who had ruled over the Medes and Assyrians. Cyrus, having taken such a prince 20 years before Babylon's capture, advanced him to be deputy king of Babylon. Hence he retained the royal title and is called "king" by Daniel. Thus Astyages (the last king of the Medes, and having no issue, according to Herodotus, 1:73, 109,127) will be this Darius, and Ahasuerus (Achashverosh) = Cyaxares (Huwakshatra), father of Astyages. Aeschylus (Persae, 766, 767) represents Cyaxares as the first founder of the empire and a Mede, and Sir H. Rawlinson proves the same in opposition to Herodotus. Aeschylus describes Cyaxares' son as having "a mind guided by wisdom"; this is applicable both to Darius in Daniel 6:1-3, and to Astyages in Herodotus. The chronology however requires one junior to Astyages to correspond to Darius the Mede and Cyrus' viceroy, whether a son or one next in succession after Astyages, probably Cyaxares...