‘In that night Belshazzar the king of the Chaldeans was slain, and Darius the Mede received the kingdom, being about sixty two years old.'
The king was probably slain in what fighting there was, along with many of his lords, but in general the Persians followed an enlightened principle of mercy in their dealings with captured peoples and encouraged them in the worship of their own gods, thanks to Cyrus himself, which was why in all probability they were welcomed by the priests of Marduk.
‘Darius the Mede'. There is no suggestion here that he was king of a separate Medan empire at the same time as Cyrus. It simply tells us that he was a Mede (and in Daniel 9:1 even more emphatically ‘of the seed of the Medes'). The so-called ‘Medan empire' of Daniel is an invention of scholars out to prove a theory. There is no evidence for it whatsoever, and it has to be reached by ignoring the clear meaning of certain other passages.
No Darius has been found in inscriptions connected with the new dawn of Babylon, but it is quite possible that he was known under another name and that Darius was a throne name. In meaning it is probably connected with the New Persian word Dara, meaning "king." Herodotus says that it means in Greek, Erxeies, coercitor, "restrainer," "compeller," "commander." We should note that the implication here is that this Darius succeeded to Belshazzar's position as ‘melek of the Chaldeans', and thus an under-king (compare Daniel 9:1). Belshazzar was not the sharru. Nabonidus was still alive.
Various suggestions have been made. One is that it was a name taken on by Cyrus when he defeated the Medes, or by his son Cambyses, to cement his position over the Medes (but the latter was certainly not sixty two years old). Another is that Darius is another name for Gubaru (Gobryas), one of Cyrus' generals, who was later appointed by Cyrus to rule Babylon. (Darius may not have ‘received the kingdom' immediately). It has been suggested that Gubaru is possibly a translation of Darius. The same radical letters in Arabic mean "king," "compeller," "restrainer." This was a different man from Ugbaru, the governor of Gutium and Persian commander who led the assault against Babylon and died shortly afterwards, but we do not know how old Gubaru was.
A connection with Cyrus could be supported by the fact that Cyrus was related to the Medes, was about sixty two years old when he conquered Babylon, and by the reading ‘in the reign of Darius, that is in the reign of Cyrus the Persian' (Daniel 6:28). This latter could, however, also support the suggestion that it was Gubaru, revealing him as under-king to Cyrus. We should note in contrast that Darius II is called ‘Darius the Persian' (Nehemiah 12:22) which may suggest that a ‘Darius the Mede' was known historically to Nehemiah.
Another explanation has been that Darius is another name for Cyaxares II, the son of Astyages, who according to the Greek writer Xenophon was Cyrus' uncle and father-in-law, and whom Cyrus might have retained temporarily as a figurehead king and have appointed over Babylon to please the Medes. It was captured by a Medan general.
But there may well be here a figure we as yet no nothing about from inscriptions. Daniel only refers to his first year (Daniel 9:1; Daniel 11:1) and then does not refer to him again for dating. He turns instead to reference to Cyrus (Daniel 10:1). This suggests that Darius may not have held the position for very long and would therefore be unlikely to be mentioned in inscriptions. His only claim to fame was his connection with Daniel.
Interestingly in the Harran stele of Nabonidus mention is made of the ‘king of the Medes' in 546 BC, four years after Cyrus became king of the Medo-Persian empire.