yesterday night one player was asking me if Green baboon's effect will be negated by Necrovalley while its in the grave. i said no baboon will still come out, but he wasnt convinced as the opinions on this seems to vary. so i just tried this on my DS and here are the results.
Field card: Necrovalley
AI field: random vanilla + mirror force + solemn warning
my monster on field: Dark Dog 1900 attk vanilla
my grave: green baboon only
all other places: 0 cards.
unexpected result 1: forgot to remove warning from my previous test play lol.
dark dog attacks, AI activates mirror force and dog gets destroyed.
Baboon effect activates, AI chains with warning.
BABOON GOT NEGATED AND STAYS IN GRAVE (same result with divine wrath).
result 2: this time i take out warning.
dark dog attacks, AI activates mirror force and dog gets destroyed.
Baboon effect activates, and sucessfully gets revived by itself.
hope this helps, and feel free to make any comments