Imperial Tombs of Necrovalley say that you can negate a spell, trap or monster's effect while Necrovalley and a Gravekeeper's monster is on the field. However, Necrovalley states that cards in either player's graveyard cannot be affected by effects, yet I was able to negate Sangan's (and the like which activates in the graveyard) effect with Imperial Tombs of Necrovalley while Necrovalley was active. Correct me if I'm wrong.

However, Necrovalley states that cards in either player's graveyard cannot be affected by effects

http://yugioh.wikia.com/wiki/Necrovalley 

Necrovalley Effect:

All "Gravekeeper's" monsters gain 500 ATK and DEF. Cards in either player's Graveyard cannot be banished. Negate any card effect that would move a card in the Graveyard, other than itself, to a different place.

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YgoPro is using an old Card Errata for Necrovalley.

We shall try to get this updated to avoid any more confusion.

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