Once Twilight Sparkle asked whether alicorns were immortal. She didn't think too much about it at the time. Now, many years later, this question is becoming more and more important to her, as her friends are aging. When Twilight first asked, she got very little in the way of an answer. But after discovering a secret below Canterlot Castle, she finally gets the information she seeks. And it is certainly not what she expected...