To get the treasure and Broken Slab first you need to go to House Beneviento and kill Angie.
After doing that go back to village and now you can get the Broken Slab in the locked door in graveyard.
Now you need to get back to location of Beneviento's Treasure. A monster will be waiting there for you so get every gun and upgrade you can from Luke before going there.
After Killing the monster use Broken Slab to open the grave and loot the Berengario’s Chalice.