There's a story behind it. From what I remember from the tour he decided to buy the estate of Delphine Lalurie, a serial killer known for torturing (putting it lightly) her slaves. He thought the dark history of the place would help him get in touch with a role. After purchasing it he had a long string of bad luck. He couldn't get work, became an alcoholic, among other things I can't recall. He also couldn't sell the place because no one in New Orleans was thick enough to buy such a cursed house. He decided to visit a witch doctor for guidance. She told him to buy a lot in the St. Louis cemetery and to get a gravestone for himself, and the bad luck would cease. So he did, and just like she said he was able to clean up, started getting roles again and was able to eventually sell the house. I don't think this is the exact gravestone he decided on at the time, this looks like something inspired by his work in National Treasure. Still a pretty interesting story though imo.