Twinkling Twilight

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Long dead ancestor of Twilight, she was the result of the tryst between Celestia and Clever Clover, and the first ever mortal alicorn. Celestia had never paid heed to the rules of mortals and gods interactions, and found much too late the law "The foals of mortals beget mortals, unless chosen by harmony." It was too late, and Twinkling Twilight was born. Celestia was not able to handle motherhood, and the death of Clever Clover opened her eyes to the mortality of her subjects. She could not stare in to the eyes of her child without seeing her old and frail, and as such, Twilight became withdrawn and depressed, her mother unable to feel love for her child for fear of her death.
She disappeared for two years before returning, a hoof lost. She spoke of an evil Draconequus who challenged her to a game of wits, and took away her hoof and replaced his own with hers. she finally found love and had a child, who's childs decendent was Star Sparkle.
And we know who was Star Sparkles child.

She still has her hoof in this pic for reference.