"The Decoy Duck" is a story by Harry Turtledove, originally published in 1992 in After the King: Stories in Honor of J.R.R. Tolkien, edited by Martin H. Greenberg. It was reprinted in Counting Up, Counting Down in 2002. It is a prequel to the Videssos Series. Turtledove states in "Counting" that it takes place 900 years before The Misplaced Legion.
It is the story of Skatval the Brisk and Kveldulf. Skatval is the chieftain of a band of Halogai who must deal with a band of priests of Phos from the Empire of Videssos. What makes these missionaries different from earlier ones, and more dangerous to Skatval's way of thinking, is the inclusion of Kveldulf, an Haloga who worships Phos.