|After completing Olberic's Chapter 4||Riverford||9,000 Leaves, Harald's Sword|
You have to complete Olberic's Fourth Chapter in order to unlock this Side Story. Travel to Riverford and you'll find Reggie on the western side of the main square. Speak to him twice to learn that Harald has been selling his precious family heirlooms to help rebuild Riverford, with Reggie mentioning that he'd like to at least keep those. The necklace, specifically mentioned, is located in Wellspring and you'll probably want a fairly strong H'aanit or Olberic. In the northern half of the town, there is a guard blocking the door to a building.
You will need to Challenge/Provoke him to get out of the way, although he has a strength of nine, so he might not be a pushover. Being weak to spears, you can use the Warrior's Thousand Spears to bring his shields down faster. Defeat the guard, then head inside to find three merchants. The one along the northern wall has a Memorial Necklace on him, so Steal/Purchase it, then bring it back to Reggie in Riverford to finish the quest.