Sickening Shoal is a rare Instant — Arcane card from Betrayers of Kamigawa (BOK), collector number 82. Current market price: $8.98 (foil $85.00), TCGplayer market prices via Scryfall.
You may exile a black card with mana value X from your hand rather than pay this spell's mana cost. Target creature gets -X/-X until end of turn.