Nightmares

During the events of the Eternal Night, the entrance of the Nychtaur into the Prime Material Plane exerted a pull on the souls of the unicorns that resided in the Dreamrealm of Valnurana. The result was a series of tears between the Dreamrealm and the Prime Material Plane through which the souls of the unicorns were able to emerge as nightmares. When the Nychtaur was subdued and claimed by Valnurana as the fulfillment of Her ancient pact with Thoth, all but one of the nightmare herd returned with the stallion to the Dreamrealm.