Duel for Dominance is a common Instant card from Innistrad: Double Feature (DBL), collector number 184. Current market price: $0.21 (foil $0.36), TCGplayer market prices via Scryfall.
Coven — Choose target creature you control and target creature you don't control. If you control three or more creatures with different powers, put a +1/+1 counter on the chosen creature you control. Then the chosen creatures fight each other. (Each deals damage equal to its power to the other.)