We've discussed in many ways how Mallory is obviously meant to be a powerful folly against Michael. While we don't know exactly what the source of her abilities are — whether she's the angelic foil or the second coming of Christ — we do know that she will play a huge part in however this story ends.
A popular theory about how that will come to be involves Mallory's power of reversing time. As we witnessed in episode five, "Boy Wonder," Mallory is able to resurrect and reverse the ageing process of a fallen doe. The theory is that using this power, the Coven will go back in time and prevent Michael from either being conceived or being born. While this seems like a tidy fix, it begs the question of how exactly they would prevent his birth, what would happen to the Harmons and Murder House, and — most importantly — would that actually prevent the apocalypse from ever happening?