Subterror Behemoth Stygokraken

FLIP: You can target Set cards on the field, equal to the number of "Subterror Behemoth" monsters you control; destroy them. You can only use this effect of "Subterror Behemoth Stygokraken" once per turn. When a face-up monster you control is flipped face-down, if you control no face-up monsters: You can Special Summon this card from your hand in Defense Position. Once per turn: You can change this card to face-down Defense Position.