Sorcery, G (1)
Common
Target creature you control fights target creature you don't control. (Each deals damage equal to its power to the other.)
"You don't find many old werewolf hunters."
—Paulin, trapper of Somberwald
Illus. Dave Kendall

7/1/2012: If either or both targets are illegal when Prey Upon tries to resolve, no creature will deal or be dealt damage.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Extended (Type 1.X)
Legal in Standard (Type 2)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern