From the Super Mario Wiki
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The Graveyard is located north of the Boneyard; it is an enclosed rectangular area with eight tombstones that occupy most of the room. At the top-center of the location is a small slope with a few steps leading to the main headstone, which Bogmire rests under. Two torches appear on each side of the tombstone, and a Fire Elemental Ghost can spawn from either one.
A single Red Jewel is the only treasure in the Graveyard. Luigi can obtain it from the downspout at the bottom-right corner. Some tombstones may release a Heart if Luigi taps them, but the top-left one always releases a large Heart that restores up to 50HP.
After Luigi vacuums up Spooky in the Boneyard, he must scan Spooky's kennel for it to make a small gust of wind dragging him into the Graveyard (similar to entering the Hidden Room from a mouse hole in the Butler's Room).
After entering the Graveyard, Luigi must tap a gravestone directly below the main one to summon three Mr. Bones (or five in the PAL Hidden Mansion). After capturing each of them, a blue aura glows around the main headstone. Luigi must walk to it, causing Bogmire to rise from it. Luigi is transported into an arena to fight the boss.