Effect
This card on the field is unaffected by all other card effects, as long as "Necrovalley" is also on the field.
The monster effect "(1): While this card is in the Monster Zone and 'Necrovalley' is on the field, this card is unaffected by other card effects" is a Continuous Effect.
"Gravekeeper's Heretic" is unaffected by both your own card effects and your opponent's card effects.
This effect applies not only when "Necrovalley" is in the Field Zone, but also when the effect of "Gravekeeper's Priestess" is applied and the field is treated as "Necrovalley".
"Gravekeeper's Heretic" is also unaffected by the effects of "Necrovalley", so its ATK and DEF will not be increased by the effect of "Necrovalley".
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