Twilight Sparkle finds herself being led to question things she's never questioned and try things she'd never tried by Rarity. But slowly Twilight begins to feel like she's being corrupted, and begins to wonder if Rarity is a vampire.
In order to save her friends from death, Twilight sought out a means to give them immortality. However, Twilight didn't know the fate she would bestow upon them would be worse then death.