Graveyard Zone, Crypt
This will probably be one of the last artifacts, if not items in general, you will get in your journey because of the amount of effort that goes into getting it. You will find four treasure maps that will show a grid (4x4 with missing holes in it) relating to a specific part in the Graveyard. Behind a locked door is the crypt, where all four maps are used. It doesn't matter what door you come out of as you're doing this; merely the correct door entered, in the right order, is what you need to get to the next part.
The Crypt is a room with four different sides, each bearing different doorways on each side. The order to go in, and the side you want... (I do wish I had the images myself, rather than trying to rip them off someone else's pages). I'll just use ASCII art to show you the correct order. [ I ] will indicate a doorway, whereas [ X ] will indicate a wall, and [ O ] will be the correct door.
Look for this side first, should be to the left(or maybe right) of the entrance:
[ I ][ I ][ X ][ I ]
[ X ][ I ][ I ][ I ]
[ I ][ I ][ X ][ X ]
[ I ][ X ][ I ][ O ]
4th row, 4th door is the first correct door.
Next side, turning the room around twice:
[ X ][ I ][ I ][ I ]
[ I ][ X ][ I ][ X ]
[ I ][ I ][ O ][ I ]
[ I ][ I ][ X ][ I ]
3rd row, 3rd door is the second correct door.
Next side, the entrance into the Crypt:
[ I ][ I ][ I ][ X ]
[ I ][ X ][ O ][ I ]
[ X ][ X ][ I ][ I ]
[ I ][ I ][ I ][ X ]
2nd row, 2nd door (the second door, but third in the order of the actual slots)
Lastly, behind the entrance:
[ I ][ O ][ I ][ I ]
[ I ][ X ][ I ][ I ]
[ I ][ I ][ X ][ I ]
[ X ][ I ][ I ][ I ]
1st row, 2nd door
This will take you to a bloody lake on a skull island. The chest you seek is far off in the distance, unreachable by shifting. Instead, you must actually swim across the water to get to it and open it up. Shifting until the skull looks to the right/left, head off in that direction to reach it. This will award you the cubed Skull artifact by opening it.