While a "Gravekeeper's" monster(s) and "Necrovalley" are both on the field, monsters your opponent controls lose 500 ATK/DEF. Once per turn, during the Main Phase, if you control no card in your Field Zone: You can activate 1 "Necrovalley" directly from your hand or GY. If this card in your possession is destroyed by an opponent's effect and sent to your GY: You can Set 1 "Necrovalley" Spell/Trap directly from your Deck, except "Necrovalley Temple".
When no Field Spell is present Gravekeeper’s Priestess by herself can fulfill the condition for Necrovalley Temple’s debuff effect.
Gravekeeper decks have a lot of swarm effects so having a Gravekeeper present would not usually be a problem. Aside from that, their boss monsters Gravekeeper’s Supernaturalist and Gravekeeper’s Visionary have amazing staying power.