You'll know you've activated all the gravestones when one of the items nearby can accept an item (the hand icon appears when you move over it). The item is in one of the corners of the graveyard. Simply run to every corner and move your cursor along it, once you see the hand, you know you've found the right spot. Then simply drop a normal, ordinary, run of the mill dagger onto the spot and it will stick inside it, thus activating the mushroom circle.
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