Ahasuerus was the King of Persia who according to the Book of Esther ruled from India to Ethiopia. He is believed by 19th century commentators to be Xerxes I of Persia. Achashverosh, as his name transliterates from Hebrew, was also mentioned in the books of Ezra, Daniel and Tobit.
The most famous appearance of Ahasuerus was as a character in the Book of Esther, in which he searches for a new queen after the death of his wife Vashti and is nearly tricked by his advisor Haman into allowing the Jewish people to be destroyed. He ends up marrying a Jewess, Esther, and appoints her uncle, Mordecai, as his counselor. Together they ruin Haman's plans and save the Jewish people.
This story would later become the basis for the Jewish holiday of Purim.
Ahasuerus generally is portrayed as vacillating, lacking in character, and easily swayed. Ahasuerus is also said to have desecrated the Temple vessels and priestly robes at the feast he made for all the provinces of his kingdom. However, the positive aspects of his personality are also emphasized. He is depicted as one of the few kings in history who ruled over the entire earth. Before his death, Nebuchadnezzar had placed all the treasures of the world he had looted in a ship, and sunk it in the Euphrates to prevent anyone finding them. God, however, had revealed their location to Cyrus when He gave orders that the Temple was to be rebuilt.
Sources: Encyclopaedia Judaica. © 2008 The Gale Group. All Rights Reserved; Robert J. Littman, The Religious Policy of Xerxes and the "Book of Esther"