Open the chest in the Black Skeletal Reaper's room
In the October 29 full-game preview, Charming Jar is inside the chest in the coastal stone crypt, in the room occupied by a Black Skeletal Reaper. Find the crypt's short stone-step entrance, go inside past the large wooden crate and bones, then open the chest. Check the treasure list for Charming Jar.

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Find the short stone-step entrance
Start: the outside of the sea-side crypt shown above. This is a local route from that building.
Follow the narrow path around the crypt and look for the opening with the short, uneven stone steps shown below. Enter through this opening and follow the steps into the building.

Find the chest beside the Black Skeletal Reaper
Inside, use the large wooden crate and skeletal remains as your next landmark. Continue into the burial room and look for its wooden chest. The enemy's health bar identifies it as Black Skeletal Reaper.
Move close to the chest and interact to open it. In the recorded pickup, the Reaper is still alive when the chest is opened, so defeating it is not shown as a prerequisite. It can attack while you are nearby.

Check for Charming Jar in the treasure list
Done when: the inventory lists Charming Jar. Its icon is a small jar, and its description identifies the contents as dried spices from an ancient land.

Charming Jar is a treasure. The Jar Club is a separate weapon: a clay jar attached to a stick. If you are looking for the weapon or the Symbol of Jigeki left after it breaks, follow that guide instead.