If this card is in your hand or GY: You can Tribute 2 WATER monsters; Special Summon this card. You can only use this effect of "Citadel Whale" once per turn. If this card is Special Summoned: You can Set 1 "Sea Stealth Attack" directly from your Deck. Once per turn, when your opponent activates a card or effect that targets 1 WATER monster you control (and no other cards) (Quick Effect): You can negate the activation, and if you do, destroy that card.
