Cankerous Thirst is a uncommon Instant card from Eventide (EVE), collector number 116. Current market price: $0.12 (foil $0.34), TCGplayer market prices via Scryfall.
If {B} was spent to cast this spell, you may have target creature get -3/-3 until end of turn. If {G} was spent to cast this spell, you may have target creature get +3/+3 until end of turn. (Do both if {B}{G} was spent.)