Cyaxares was the king of the Medes, who helped destroy the mighty and terrifying Neo-Assyrian Empire, create a massive empire based in the small region of Media, and laid the foundation of the Achaemenid Persian Empire.
T. Cuyler Young, Jr., Cambridge Ancient History, The Early History of the Medes and the Persians and the Achaemenid Empire to the Death of Cambyses
Cambridge History of Iran, The Medes and the Neighboring Countries
Kevin Lelous, The Battle of the Eclips (May 28, 585 BC): A Discusssion of the Lydo-Median Treaty and the Halys Border
Encyclopaedia Iranica, Cyaxares