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An entropy engine was a Dalek weapon used during the Last Great Time War. It created a bubble around planets, speeding up time inside, and making everything crumble to dust within minutes. The resulting energy was sucked back into the engine, and was recycled behind impenetrable flux shields. This energy was enough to power an entire time-armada. The Ninth Doctor believed this was "Dalek efficiency at its best". The Doctor believed the engine should have burned with everything else. The Unon had come into possession of an entropy engine, and built their base, the Perpetual City, around it. They used it to stop the Excrothians' time travel experiments by destroying Excroth.
The surviving Excrothians rebranded themselves as "The Lect" and began a war against the Unon. After befriending Rose Tyler, the Lect managed to have the Ninth Doctor bring Grand Prelon Marleth to the heart of the entropy engine where a Unon shot him down. While the Doctor managed to have the Unon stand down their crusade, Marleth, with the last of his strength, shot the engine, causing it to explode. (COMIC: Weapons of Past Destruction)
The Unon's usage of the "fabled entropy engine" was detected by the nascent New Dalek Paradigm but they were content to allow the Doctor to deal with the upstarts. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)