It's called Ponies, Demons and Spike. Blog Here

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That idea doesn't make much sense. For killing Shining Armor and Cadance, Twilight would likely go insane with rage, the type that would make what she did with Tirek look like child's play, even if she didn't have the other of the other Alicorns. It ignores how the rest of the empire would act. And Celestia, Luna, and Discord would probably want them destroyed too.

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Like I said, not all the details have been thought up yet. It's basically a first draft. and Celestia want Twilight to kill the demons. As for Luna and Discord, Luna has no Idea who Cadence is and was friends with Quartz, something which is odd when Celestia never mentions her, and Discord has is own agenda involving the missing demon Baphomet.