Twilight devises a way to end a certain threat against Equestria, once and for all. She attempts a powerful, dangerous spell and, unfortunately, everything goes exactly as she plans. Her actions aren't driven by the purest of motives, and her disillusionment leads her down the road to dire consequences. . . a road somepony has walked before her.
Featured on Equestria Daily: http://www.equestriadaily.com/2012/06/ytpmv-ponponpony.html#idc-container (With the weirdest URL ever!)