The Celtic Goddess Brigid tasked nineteen nuns with tending her sacred flame. So long as the flame burned, the veil between this world and the Otherworld would remain intact.
Each night, a nun took a turn tending the flame. On the twentieth night, the Goddess herself would return to keep watch. Nuns were selected based on years of service, their dedication to the Druid order, and their use of protective Light Magic in order to maintain the veil.