The locked casket is an item obtained during the You Are It quest. It is obtained by digging up Duchess Oriana's grave in the Lumbridge cemetery. After the player finds the casket key in Draynor Manor, it's opened to reveal the old necklace.
If the player destroyed the necklace after the quest, they do not need to retrieve the casket key from Draynor Manor. They must simply return to Duchess Oriana's grave and dig to retrieve the old necklace.
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