You can only have 1 'Gravekeeper's Chief' on your side of the field. As long as this card remains on the field, your Graveyard is unaffected by the effect of 'Necrovalley'. If you Tribute Summon this card successfully, you can Special Summon 1 Monster Card that includes 'Gravekeeper's' in its card name from your Graveyard to the field.