Letters

The Elements of Harmony have rules about the Celestial prison. It is not; after all, simply a place of holding. It is a place of rehabilitation as well.

Nightmare Moon may be stuck in the Celestial prison but that doesn't mean that she won't be rehabilitated. After all she needs to be civil with her Co-Regent doesn't she? But how can she and Celestia be civil if they are millions of meters apart. Simple, letters.