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. (COMIC: Weapons of Past Destruction)

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