When your opponent activates a card or effect (Quick Effect): You can send this card from your hand or field to the GY, then target 1 "Subterror" monster you control; negate the activation, then change the targeted monster to face-down Defense Position. You can target 1 face-up monster you control; change it to face-down Defense Position, and if you do, Special Summon 1 "Subterror" monster from your hand or GY, in face-up or face-down Defense Position. You can only use each effect of "Subterror Fiendess" once per turn.