When Slith caused a triple eclipse of the sun and began the Eternal Night, a new horror was visited upon the people of Sapience. Every night at midnight a powerful and monstrous being known as the Nychtaur would appear in a randomly chosen city. It would rampage through the streets, slaying any it came upon, and often took huge parties of warriors to dispatch.
The beast's origin was eventually revealed. Uncounted centuries earlier, after the conclusion of the War of Humanity, the Goddess Valnurana mourned the lost race of unicorns that had been destroyed in the final battle. She had made a bargain with the God Thoth, offering Him something unknown in exchange for allowing the souls of the unicorns to reside with Her in the Dreamrealm. As a condition of Their bargain, however, Thoth kept to Himself the soul of a single male, promising to yield it to Her when "the night swallows up the sun and moon."
The events of the eternal night fulfilled that condition, and the terrible Nychtaur was, indeed, the soul of this ancient unicorn stallion. When subdued by the net of stars obtained during the course of the events of the Vanished Moon, the bull-like demon resumed its old form and led the spirits of the other unicorns, the nightmares, back to the Dreamrealm.