The Thirst of Ages, or Methuselah's Thirst, is a characteristic of Methuselahs and other ancient Kindred. After many years of being a Vampire, the blood of mortals will not be sufficient to feed and sustain a vampire's thirst. Thus, the thirst can only be slaked by supernatural (typically vampire) blood. Diablerists are the most notorious bearers of this thirst; it can be hard to say whether serial diablerie makes one more likely to incur this curse, or if the thirst for Kindred vitae almost necessarily leads one to diablerie.
