Evil Empire
An empire (now dissolved) that once spanned a bigger part of the Disc including Überwald as well as a number of smaller countries such as Borogravia and Mouldavia (according to Night Watch), with the double-headed bat as symbol, and once ruled by a Sourcerer known as the Evil Emperor, who was, well, evil. Under the Evil Empire, all sorts of nasty things were created and used in war. As John Hicks puts it in Unseen Academicals, "you got what it said on the Iron Maiden".
It was mentioned under the names Unholy Empire and the Dark Empire in the earlier boobs, and first described in detail in Unseen Academicals.
It is an allusion to the Soviet Union (confirmed by Terry) with elements of standard fantasy Dark Empires, like Tolkien's Mordor.
See here for more.
Annotation
In the middle 1980's at the height of the cold war, American president Ronald Reagan famously described the Soviet Russian-headed Warsaw Pact of nations as "the Evil Empire". Some unkind speculators wondered if the title had been provoked by Ronnie's latest viewing of "Star Wars", or by his flirtation with evangelical Christianity in which godless Communism was always equated with the Antichrist.
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