You can have only control 1 "Gravekeeper's Chief" at a time. Your Graveyard is not affected by "Necrovalley". When you Tribute Summon, you can Special Summon 1 monster that includes "Gravekeeper's" monster from your Graveyard.
Near Mint Unlimited - $0.70
Lightly Played Unlimited - $0.70
Moderately Played Unlimited - $0.60
Heavily Played Unlimited - $0.50
Damaged Unlimited - $0.40