Actually, to be a little more specific, Necrpvalley doesn´t "lock up" the graveyard like some people think.
Necrovalley prevents cards OUTSIDE the graveyard to affect cards INSIDE the graveyard, and prevents players from removing cards in the graveyard from play. Simple as that.
There are cards whose effect SOUND like they should be affected by "Necrovalley" but aren´t. Those are:
1) Cards whose effect activate when they themselves are sent to the graveyard. Mystic Tomato and the other "attribute searchers", Sangan, Archfiend of Gilfer, Dark Necrofear (assuming you summon her BEFORE Necrovalley is activated or while it´s being negated), Witch of the Black Forrest, etc resolve just fine because they don´t target cards in the graveyard. Instead, they have effects activated INSIDE the graveyard, targeting cards in the DECK.
2) Cards that target THEMSELVES while they are in the graveyard. Sinister Serpent, Vampire Lord, etc activate themselves and Giant Germ, Nimble (and the new) Nomonga, Marie The Fallen One, Makyura The Destructor and Helpoemer target the players, and Monsters whose tribute themselves (and send themeselves to the graveyard) to activate their effect, like Exiled Force and, their names escape me at the moment but one can be tributed while face-upon the field to negate the activation of a Spell and the other the activation of a trap.
3) Cards whose effects activate BEFORE they hit the graveyard. For example, Slate Warrior´s effect (reducing ATK and DEF by 500) activates when he´s destroyed in battle, but BEFORE he hits the graveyard.
So be careful when Necrovalley is active on the field. Try to pay close attention and ask yourself, "Is this a card OUTSIDE the Graveyard trying to affect a card(s) inside the graveyard.
Waldo10032 - The Fiendish Duelist
Level 2 Judge
Level 1 Tourney Organizer
"Know Your Man. Know His Sword"