Cyaxares (Babylonian Umakishtar) in Wikipedia
yaxares or Hvakhshathra (Old Persian: 𐎢𐎺𐎧𐏁𐎫𐎼 Uvaxštra, Greek Κυαξάρης; r. 625 - 585 BC), the son of King Phraortes, was the first king of Media .
He reorganized and modernized the Median Army, then joined with King Nabopolassar of Babylonia. This alliance was formalized through the marriage of Cyaxares daughter, Amytis with Nabopolassar's son, Nebuchadnezzar II, the king who constructed the Hanging Gardens of Babylon as a present for his Median wife to help with her homesickness for the mountainous country of her birth.
These allies overthrew the Assyrian Empire and destroyed Nineveh in 612 BC. After this victory, the Medes conquered Northern Mesopotamia, Armenia and the parts of Asia Minor east of the Halys River, which was the border established with Lydia after a decisive battle between Lydia and Media, the Battle of Halys ended with an eclipse on May 28, 585 BC.
The conflict between Lydia and the Medes was reported by Herodotus as follows:
"A horde of the nomad Scythians at feud with the rest withdrew and sought refuge in the land of the Medes: and at this time the ruler of the Medes was Cyaxares the son of Phraortes, the son of Deďokes, who at first dealt well with these Scythians, being suppliants for his protection; and esteeming them very highly he delivered boys to them to learn their speech and the art of shooting with the bow. Then time went by, and the Scythians used to go out continually to the chase and always brought back something; till once it happened that they took nothing, and when they returned with empty hands Cyaxares (being, as he showed on this occasion, not of an eminently good disposition) dealt with them very harshly and used insult towards them. And they, when they had received this treatment from Cyaxares, considering that they had suffered indignity, planned to kill and to cut up one of the boys who were being instructed among them, and having dressed his flesh as they had been wont to dress the wild animals, to bear it to Cyaxares and give it to him, pretending that it was game taken in hunting; and when they had given it, their design was to make their way as quickly as possible to Alyattes the son of Sadyattes at Sardis. This then was done; and Cyaxares with the guests who ate at his table tasted of that meat, and the Scythians having so done became suppliants for the protection of Alyattes.
After this, since Alyattes would not give up the Scythians when Cyaxares demanded them, there had arisen war between the Lydians and the Medes lasting five years; in which years the Medes often discomfited the Lydians and the Lydians often discomfited the Medes (and among others they fought also a battle by night): and as they still carried on the war with equally balanced fortune, in the sixth year a battle took place in which it happened, when the fight had begun, that suddenly the day became night. And this change of the day Thales the Milesian had foretold to the Ionians laying down as a limit this very year in which the change took place. The Lydians however and the Medes, when they saw that it had become night instead of day, ceased from their fighting and were much more eager both of them that peace should be made between them. And they who brought about the peace between them were Syennesis the Kilikian and Labynetos the Babylonian: these were they who urged also the taking of the oath by them, and they brought about an interchange of marriages; for they decided that Alyattes should give his daughter Aryenis to Astyages the son of Cyaxares, since without the compulsion of a strong tie agreements are apt not to hold strongly together." (Histories, 1.73-74, trans. Macaulay)
Cyaxares died ten years after the battle and was succeeded by his son, Astyages, who was the maternal grandfather of Cyrus the Great through his daughter Mandane of Media.