Over a thousand years ago Starswirl the Bearded wrote his last spell, an incantation that by his own admission was "broken and unfinished". Centuries later Twilight Sparkle derived from it the spell that allowed her to ascend and become an alicorn. But while she was able to reverse the effects of the spell on her friends, they weren't the only ones affected.
Co-written with River Road and Edited by The Weakest Link