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It's hard to say just how dark things might get for Earth's Mightiest Heroes by the end of "Avengers: Doomsday." Reports say the movie was described as "too dark" at test screenings, and Robert Downey Jr. apologized in advance for how bad things will get. With so little to go on about what the next big Marvel movie might have in store, all that's left to do is look to the ancient texts — also known as comic books — to try to get the inside track on what could be waiting for us on the horizon, which seems fairly doom-riddled.

What we do know is that on December 18, the MCU will be broken to bits by one very bad man in an iron mask, and there are so many ways his plan could come to fruition.…