While a “Gravekeeper's” monster and “Necrovalley” are on the field, all monsters your opponent controls lose 500 ATK/DEF. Once per turn, during the Main Phase, if you do not control a card in your Field Zone: You can activate 1 “Necrovalley” from your hand or GY. If this card is destroyed by your opponent’s card effect and sent to the GY: You can Set 1 “Necrovalley” Spell/Trap from your Deck, except “Necrovalley Temple”.