Basically a sequel to a TCB/Shadow over Insmouth story idea I never managed to write up.

Celestia waits alone in Canterlot for the restored Human race to come after her seeking vengeance. Things don't go as she expected.

1000 years ago, Celestia took a huge gamble to exorcise her sister rather than banish her.
Now Luna rules a land without a sun, and her prize student Twilight Sparkle believes she sees omens that the power responsible for the banishment of the sun will soon return.