Ahasuerus - (Easton's Bible Dictionary)
There are three kings designated by this name in Scripture. (1.) The father of Darius the Mede, mentioned in Dan. 9:1. This was probably the Cyaxares I. known by this name in profane history, the king of Media and the conqueror of Nineveh.
(2.) The king mentioned in Ezra 4:6, probably the Cambyses of profane history, the son and successor of Cyrus (B.C. 529).
(3.) The son of Darius Hystaspes, the king named in the Book of Esther. He ruled over the kingdoms of Persia, Media, and Babylonia, "from India to Ethiopia." This was in all probability the Xerxes of profane history, who succeeded his father Darius (B.C. 485). In the LXX. version of the Book of Esther the name Artaxerxes occurs for Ahasuerus. He reigned for twenty-one years (B.C. 486-465). He invaded Greece with an army, it is said, of more than 2,000,000 soldiers, only 5,000 of whom returned with him. Leonidas, with his famous 300, arrested his progress at the Pass of Thermopylae, and then he was defeated disastrously by Themistocles at Salamis. It was after his return from this invasion that Esther was chosen as his queen.