Caustic Exhale is a common Instant card from Tarkir: Dragonstorm (TDM), collector number 74. Current market price: $0.12 (foil $0.17), TCGplayer market prices via Scryfall.
As an additional cost to cast this spell, behold a Dragon or pay {1}. (To behold a Dragon, choose a Dragon you control or reveal a Dragon card from your hand.) Target creature gets -3/-3 until end of turn.