Celestia has mourned the loss of her sister for a long time, but now that she has her back she should be happy, right? Yet, she still feels remorse for deeds of the past, even when she knows there's no sense in doing so. So when her worst nightmare and greatest dream comes true, will she be able to treat it like the threat it is, or will the land dissolve into chaos?
(Eh, if you want to know the actual ending I suggest you watch The Return of Harmony parts one and two. If you haven't already.)