A thousand years of raising both sun and moon can take its toll on a pony. There comes a point when you just need to rest.
Celestia, after over a thousand years of rule, is in need of just that. As she closes her eyes, she is presented with visions of ponies she has met, which if life had been different, she would have liked to call friends. Visions which all seemingly have something in common. Then there's the constant nagging from an all too familiar voice.
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I guess this straddles the line between sad and comedy, but as I'm not allowed to pick both, I just went with sad. Meh.