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You can use the effect of "Gravekeeper's Chief" to Special Summon the same monster you Tributed to Summon "Gravekeeper's Chief".
If you Set, Flip Summon, or Special Summon "Gravekeeper's Chief", its Special Summoning effect is not activated.
If you have "Gravekeeper's Chief" and "Necrovalley" on the field, you can use "Monster Reborn", "Premature Burial", "Call of the Haunted", etc. to revive monsters from your Graveyard. Also, if you have "Gravekeeper's Chief" on the field, your opponent can use "Monster Reborn" on monsters in your Graveyard.
You cannot activate "The Shallow Grave" while "Necrovalley" is active, even if you control "Gravekeeper's Chief", because your opponent cannot Special Summon a monster from his Graveyard.
If "Necrovalley" is on the field, and both players control a "Gravekeeper's Chief", then the Flip Effect of "Fiber Jar" can resolve normally, but if neither, or only 1, player has "Gravekeeper's Chief", then the Flip Effect of "Fiber Jar" disappears.
If an active "Skill Drain" is removed from the field while 2 "Gravekeeper's Chiefs" are face-up on the same field, one "Gravekeeper's Chief" is immediately sent to the Graveyard.