1 Solve the art room puzzle
Go to the art room in the eastern side of the mansion (1F). It's the long corridor with art images on the wall and crows making loud noises. Solve the art room puzzle to get the first death mask.
2 Solve the armour room puzzle
Go to the armour room in Mansion 2F where there are statues of warriors. Push the statue with a long ax back to its place then push the one with a short ax then do the same for the one with the shield. Click the switch and you will get a box that includes the mask.
3 Go to the plant room in 1F
Go to the plant room in the eastern wing of the mansion (1F). Place the chemical inside the pump then turn the pump on to the red side. The plant will die and you will find the 3rd mask behind it.
4 Kill the snake in 2F
Go to the south east until you find the door leading to the giant snake's room. Defeat the snake, with few shotgun shots, then get the 4th mask from the floor.
5 Go to the grave outside the mansion
Reach the graveyard outside the mansion again. The place where you placed the arrowhead to get the book of curses.
6 Place the death masks
Get down into the grave where you got the book of curses earlier. Place the four death masks onto the four graves.
7 Kill the zombie
When you place all 4 death masks, a coffin will fall into the ground and a powerful zombie will show up. Note that you need to fire many shotgun rounds for that zombie to die.
8 Search the coffin
You will find the stone and metal object inside the coffin. Note that you won't be able to leave without killing the zombie first.