The Forgotten Grave Keeper can't be the target of abilities you control as long as it's in your graveyard.

Whenever The Forgotten Grave Keeper enters the battlefield, gain an experience counter for each creature card in your graveyard.

{T}, Sacrifice The Forgotten Grave Keeper: Return target creature with mana value X or less from your graveyard to the battlefield, where X is the number of experience counters you have.