When your opponent activates a monster effect, or an opponent's monster declares an attack (Quick Effect): You can Special Summon this card from your hand. You can only use this effect of "Kuriboh - Multiply!" once per turn. Once per turn, when your opponent activates a monster effect on the field, or during damage calculation, when an opponent's monster attacks (Quick Effect): You can add to your hand, or Special Summon, 1 "Dark Magician" or 1 monster with 300 ATK/200 DEF from your Deck or GY, then you can change that opponent's monster's ATK to 0.
