Description: 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.Hooves of Clay
Additional Tags: Twilight deals poorly with disillusionment.