“Mistmane, it was always about Mistmane”, Sable Spirit thought, “Everypony seemed to only have eyes for her. Blinded by her Beauty. Even now, after she left!” The orange unicorn walked into her room, tomes and scrolls were lying everywhere. She made up her mind. She would become more beautiful than Mistmane ever would be, no matter what it would cost. She was possessed by her desire to outclass her friend. Sable had spend hour, reading and searching through text, be they new or ancient, for some clue, some hint what she would have to, but found nothing. But one night everything changed.
 
In her dream, she found herself in midst of a black void, walking seemingly on darkness itself. The echo of her hoofs on the ethereal ground rag out into the nothingness. Sable did not know for how long she had been walking… or even why much less what she was walking towards, but something seemed to draw her towards it. It was like a faint whisper, like wind through branches on a rainy autumn evening. Than she saw it. Two glowing dots appeared ahead of her. A great blue ram, bigger than her stood there motionless in the abyssal-blackness. “Who are you?”, Sable Spirit asked.
 
“I am a part of the part, which in the beginning was all.” rang his voice in the dark.
 
“That’s not really helpful, and what do you want from me?”, came her reply. “Yet that is who and what I am. but as to what i want, it is less about what I want, but what you want and what I can give. You search for beauty unmeasured, are you not? and that is what I can give you.” His Eyes glowed and so did the bell that he wore around his neck. Form the blackness came a scroll. “This is what you were looking for. Read it and when you awake, you will know what to do. Do you want me to show you?” Sable hesitated, but than the urge to overcome her rival overcame her again: “Yes! Show me!” The scroll unfurled.
 
As Sable woke up she felt light headed. She sat at her writing desk, seemingly having fallen asleep here last night while reading. Suddenly the memories of her dreams came back to her. She knew what to do. Every step and every incarnation needed. Could it be true? She set everything up and worked her magic. Suddenly there was a bright light and she was blinded as she felt the magic entering her body and beginning its work. When vision came back to her she looked into the mirror speechless at what she saw.
 
There stood the most beauty full mare she ever laid eyes on. So beautiful that she struggled to even put into words what she saw. It was as if she was radiating with beauty. She couldn’t take her eyes of herself and looked at herself in every angle possible filled with pride at what she saw.
 
Than it happened and her eyes widened and they were filled with terror and aghast in realization, as her youth and beauty withered away. It was as if one would watch a flower dry out in seconds. were moments ago stood a mare at the peak of her beauty, was now the face of and old, wrinkled and gray maned pony. than she was it. in the mirror behind her stood he. hastily she turned around to see nothing. “Don’t try to find me in the waking world.”, echoed his voice in her head. she turned to the mirror. “YOU LIED TO ME!”, she screamed at the mirror and its dark inhabitant, “You promised me beauty, beyond any mare that ever lived!”
 
“And you were, were you not? Did you not marvel your own image mere minutes ago?”, echoed his voice in her head. His grin widened: “But I never said that it would last. The hearts of your kin surely are easy to fathom, when greed and desperation become to heavy of a burden, you abandon all common sense, when presented an easy way out. You like are children. Yes the spell gives you beauty beyond measure for a short time, but at the cost of your youth. Did you never consider, when an offer seems to good to be true it rarely is? It is not my fault that you never bothered to ask for side effects!” “Why are you doing this!”, she cried and all she heard was laughter. Sable felt a headache grow, as she watched the shadow in her mirror disappear.
 
Ever wondered where Sable Spirit got the spell that made her turn old from? Well, if you can’t trust an ominous goat that spoke to you in your dreams with glowing red eyes, that makes you an offer to make all your wishes come true, then who can you trust? Well he never said that her beauty gain would be lasting… and she never bothered to ask for side effects  
(Sorry that’s longer than originaly intended, I hope you all like this little headcanon of mine)