Hunter's Mark is a uncommon Instant card from The List (PLST), collector number AFR-188. Current market price: $0.11, TCGplayer market prices via Scryfall.
This spell costs {3} less to cast if it targets a blue permanent you don't control. This spell can't be countered. Target creature you control gets +1/+1 until end of turn. Then it deals damage equal to its power to target creature or planeswalker you don't control.