Not all parts of the Dark Souls II: Crown of the Sunken King taken place in the sunken temple region. If you head back to the grave of Vendrick and hit the action button on his armor, you'll enter the Memory of the King. Depending on where you are in the story will determine what the old king says to you. Once he's done you can leave or attack him to leave, each ends in the same result. Note, you don't need to kill Vendrick to access this memory.
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