"Festival Night" is a fantasy short story by Harry Turtledove, first published in Spells of Wonder, edited by Marion Zimmer Bradley, DAW 1989. The editorial note identifies it as a horror story. It has not been reprinted.
Ricold Botron, a spoiled young man of the city-state of Cathaly, thinks he has found his heart's desire, and a perfect match for both their houses, in the form of Ellene Caffilos. Prophetic inquiries soon reveal that their love will have terrifying ramifications for the city-state at large.