Cyaxares II was said to be a king of the Medes whose reign is described by the Greek historian Xenophon. Some theories have equated this figure with the "Darius the Mede" named in the Book of Daniel. However, the fact that he is not mentioned at all in the history of Herodotus, nor in the very different history of Ctesias, has caused many scholars to debate whether such a king ever actually existed.
According to Xenophon's Cyropaedia, Cyaxares II followed king Astyages to the throne of the Mede Empire, and was also brother of Mandane, Cyrus the Great's mother. He describes the Persian Cyrus as cooperating with his uncle, Cyaxares, in order to conquer Babylon in 539 BC. However Cyaxares was by then an old man, and because Cyrus was in command of the campaign, the army came to regard Cyrus as king. Cyrus thus received not only the king's daughter (his first cousin), but his kingdom, as dowry, and the aged Cyaxares became Cyrus' viceroy in Babylon for two years until his death, when Cyrus seized that kingdom as well.
"4. That Darius the Mede was Cyaxares II, the son of Astyages. Compare the statements in (Prophets and Kings 523, 556, 557) concerning Cyrus as the nephew and general of Darius with Xenophon’s claim that (1) Cyrus, Astyages’ grandson through his mother Mandane, had become acquainted with his uncle Cyaxares during the years Cyrus spent at the court of his Median grandfather (Cyropaedia i. 3. 1; 4. 1, 6–9, 20–22; 5. 2) ; (2) that Cyaxares followed his father on the throne as king of Media, after the latter’s death (i. 5. 2); (3) that when Cyrus had conquered Babylon he visited his uncle with gifts and offered him a palace in Babylon; that Cyaxares accepted the presents, and gave Cyrus his daughter as well as the kingdom (viii. 5. 17–20). Although the details of the story as given by Xenophon cannot be accepted, it is possible that the Greek writer preserves correctly the tradition that Cyaxares was the last Median ruler, and that he was Cyrus’ father-in-law as well as an intimate friend of the great Persian. If these points can be accepted as historical facts, it can be assumed that Cyrus, upon rebelling against Astyages, permitted Cyaxares to rule as a shadow king to please the Medes. At the same time everyone in the kingdom would know that the actual sovereign was Cyrus, and that Cyaxares was a mere figurehead. In that case Darius the Mede may be identified with Cyaxares II, who, presumably, had come to Babylon at Cyrus’ invitation to act in an honorary capacity as king. That Cyaxares II was advanced in age at the time of the fall of Babylon can be shown as follows, assuming Xenophon to be correct: Cyaxares II was the father-in-law of Cyrus. Cyrus himself was most likely at least 40 years old at the time, as is evident from the fact that his son, Cambyses, was mature enough to represent him in an official position during the next New Year’s Day activities. Hence Cyaxares II could have been 62 years old at the fall of Babylon, the age Daniel assigned to Darius the Mede. His comparatively advanced age—in a time when most people died young—may have been responsible for the fact that he did not survive the fall of Babylon very long. This would explain why Daniel mentions only his first regnal year. Xenophon reports nothing further concerning Cyaxares shortly after the conquest of Babylon. Daniel’s statement that Darius was the “son” of Ahasuerus should probably be understood as meaning that he was the “grandson” of Ahasuerus. That the Hebrew word for “son” may mean “grandson,” or an even more remote descendant, can be abundantly demonstrated (see on 2 Kings 8:26). The English form Ahasuerus is from the Heb. ’Achashwerosh, which might possibly be a rendering of Uvaxshtrah, the Old Persian spelling of Cyaxares I, but not of Astyages. If after his arrival at Babylon, Darius became a special friend of Daniel’s, it is understandable that the prophet would date the visions received during this brief reign in terms of Darius’ regnal years ( Daniel chs. 9:1; 11:1), rather than of the regnal years of Cyrus. However, after the one year credited to Darius, Daniel dated events in terms of the years of Cyrus’ reign (Daniel chs. 1:21; 10:1). Contemporary evidence that might shed light on this reconstruction of the history of Cyaxares II is ambiguous and meager. There is a possible reference to Cyaxares in the Nabonidus Chronicle. Since it is certain that Gubaru lived for many years after the conquest of Babylon, whereas Ugbaru died soon after, and a state mourning was provided for some high personage during the same month, it may be possible to see Cyaxares II in the Ugbaru of the Nabonidus Chronicle. Or, the name of Cyaxares may have been in the broken line which speaks about the death of a distinguished individual for whom a nationwide mourning was held. However, there seems to be an error in the first mention of Ugbaru in the Nabonidus Chronicle. Either the name Ugbaru is a scribal error for Gubaru, or the title “governor of Gutium” was by mistake transferred by the author of the tablet from Gubaru to Ugbaru. A second possible piece of contemporary evidence may lie in the double mention of a Cyaxares in the great Behistun inscription of Darius I (on the Behistun inscription see Vol. I, pp. 98, 110). Among the several pretenders to the throne against whom Darius I fought were two who claimed to be of the family of Cyaxares. The Cyaxares in question may have been either Cyaxares I, the father of Astyages, or possibly Cyaxares II, the father-in-law of Cyrus, and last shadow king of Media. The foregoing summary makes evident that there are still many obscure factors in the solution of the problem of identifying Darius the Mede from historical and archeological sources. All things considered, however, this commentary favors the fouth view." 
^Nichol, Francis D.: The Seventh-day Adventist Bible Commentary : The Holy Bible With Exegetical and Expository Comment. Washington, D.C. : Review and Herald Publishing Association, 1978 (Commentary Reference Series),