All "Gravekeeper's" monsters gain 500 ATK and DEF. Cards in the Graveyard cannot be banished. Negate any card effect that would move a card in the Graveyard to a different place. Negate any card effect that changes Types or Attributes in the Graveyard.
Necrovalley is able to boost all your Gravekeeper monsters’ stats by a significant amount. Additionally, Gravekeeper cards can specifically bypass Necrovalley’s graveyard disruption.
Being the center of Gravekeeper decks, there are lots of ways to get Necrovalley from your deck and graveyard.
While Necrovalley is face-up you can Special Summon Gilasaurus without letting your opponent revive a monster.