Necrovalley: The Greatest Field Spell of All, but SO Many Questions and Mythos...
If My Opponent's Manticore Of Darkness is Destroyed and sent to the graveyard, and at the end phase they discard a beast, beast-warrior, or winged-beast; will it return since the effect is targeting itself? Or does the outside card that must be discarded affect the whole effect where as it's not just targeting itself?!

Ahh, the Necrovalley questions just never end. The following rulings come straight from netrep:

"Necrovalley" will NOT negate effects that are activated when a card is sent TO the Graveyard or that activate IN the Graveyard or that count cards in the Graveyard. So it will NOT negate the effects of "Sangan", "Witch of the Black Forest", "Black Pendant", "Giant Rat", "Mystic Tomato", "Kuriboh", "Graceful Charity", "Sinister Serpent", "Marie the Fallen One", "Buster Blader", "Shadow Ghoul", "Helpoemer", "Sword of Deep-Seated", "Vampire Lord", "Coffin Seller", and "Magical Thorn".

"Necrovalley" will NOT negate effects that do not target a card or cards in the Graveyard. So "Necrovalley" will NOT negate the effects of "Dark Necrofear" (when destroyed and sent to the Graveyard), "Revival Jam", "Winged Sage Falcos", "Twin-Headed Behemoth", "Mystical Knight of Jackal", and "Red-Moon Baby".

So in other words, no, Necrovalley won't stop Manticore since it falls into the same category as Twin-Headed Behemoth, Dark Necrofear, Vampire Lord etc. The effects of these monsters do not target themselves.