Celestia prides herself on preparedness. She expected the return of Nightmare Moon, putting measures in place to ensure her sister's return to her right mind.

Court thaumaticists theorized that the breaking of the Lunar Binding Charm would result in the release of Discord. So Celestia ensured that he could be resealed with comparative ease.

She put measures in place to warn her when Sombra's prison was weakening.

But there is another foe from her past, whose hostility she refused to acknowledge. And he is coming.

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This story begins shortly before 'Keep Calm and Flutter On' and follows canon up to but not including that point.
'Gore' in this story is limited to the prologue and the chapter 'Return to Canterlot part 2', and is very brief (and in my opinion minor) in both occurrences.