Midrash su Esdra 4:1

And it came to pass in the days of Ahasuerus. R. Levi said: "He was the brother of a head and the coeval associate of a head. The brother of a head, i.e., the brother of Nebuchadnezzar, the wicked, who was called head, as it is said (Dan. 2, 38) Thou art the godhead. And the coeval associate of a head — what Nebuchadnezzar had done, he had intended to do. Nebuchadnezzar had killed, he had intended to kill; Nebuchadnezzar had destroyed; he had the intention, and so reads the passage (Ezra 4, 1) In the beginning of the reign of Ahasuerus, they wrote slanders on the inhabitants of Judea and Jerusalem." Samuel said: "[Ahasuerus has the meaning of black i.e.] in his days the faces of the Jews were black as the bottoms of pots." R. Chanina said: "[Ahasuerus has the meaning of 'woe to his head' (Ach rosh). Whoever mentioned his name said 'Woe to his head.' " R. Jochanan said: "[It has the meaning of 'poor' (rosh)] His taxes were so heavy that men became poor, as it is said (Est. 9, 1) And King Ahasuerus imposed a tribute upon his land." (Ib. 1)

"I shall return vengeance to My foes": These are the Cutheans, of whom it is written (Ezra 4:1) "And the foes of Judah and Benjamin heard that the people of the exile were building a sanctuary, etc."