Subterror Behemoth Dragossuary

FLIP: You can activate this effect; this turn, "Subterror" cards you control cannot be destroyed by your opponent's card effects. You can only use this effect of "Subterror Behemoth Dragossuary" 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.