Millenniummon is a Composition Digimon that resulted from the fusion of Mugendramon and Chimairamon, absorbing data from multiple powerful Digimon in the process. The combination gave it an overwhelming amount of power—so much so that it's basically considered unbeatable. How exactly it came to be fused together remains a mystery. Researchers have looked into it, but honestly, they're not expecting to turn up any real answers at this point.
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