This Mirror Left Blank

Sunset Shimmer considers Twilight Sparkle to be a very close friend. It's impressive, considering that the two of them haven't actually met.

Every alicorn has a mirror, meant to hold her if she ever turns upon Equestria in rage and bitterness. She will be safe inside, comforted by the familiar.

Twilight has come to Canterlot to update her own personal prison. While she's there, an old friend wishes to meet her for the very first time.