This story is a sequel to Kindness and the Heart of Shadow
Shortly after the events of Kindness and the Heart of Shadow, Fluttershy begins to worry how much her foal might take after its father—the evil King Sombra. While Twilight and Pinkie Pie head for the Crystal Empire to learn more about the shadow ponies, Fluttershy comes to a realization that raises the question of immortality. Discord is so afraid of the thought of losing her that he flees, only to come to a terrifying realization of his own...
A Fluttercord story, with a couple other ships thrown in for good measure.