Carnot the Eternal Machine

Phantom Nightmare · Single card

Cannot be Normal Summoned/Set. Must first be Special Summoned (from your hand or GY) during a Duel in which your opponent has activated a monster effect in their hand or GY. You can only Special Summon "Carnot the Eternal Machine" once per turn. Once per turn, if your opponent activates a monster effect (except during the Damage Step): You can make this card gain 1000 ATK. If this card is sent to the GY, except from the hand or Deck: Shuffle this card into the Deck.