The fourth book in the Who Rules? Saga.

Luna and Celestia have decided to take a trip to get away from it all. Which means another princess is going to have to step up and take the reigns in Canterlot. Surely, nothing bad could happen while they were gone. Either to them or to Canterlot. (Hint: It happens to both groups.)

WARNING: This story contains graphic depictions of sex and powerplay. If you are offended by those things, do not read this.

You might also want to read the three previous Who Rules? stories to understand everything that is happening.