Canterlot, after the great wedding.
The fight with Queen Chrysalis has left Celestia bereft of any energy, for an unkown spell the Queen wove into her attack drained it all out of her, true to the Changeling's parasitic nature. As she lies at the brink of death, an old pact becomes effective, a pact to ensure the Princesses' safety and survival.
But every pact has its price. And the force behind it is not one to be bargained with.

A/N: A great many thanks to my friends Novelty and Flynt for the proofreading and the much needed encouragement!