This piece of jewelry was owned by Morgana. It's possible that it was an artifact of the Old Religion.
When Agravaine came to her hut and warned her that Arthur wanted to cure his father with the help of a sorcerer, she cast an enchantment on this pendant. She put it in her fireplace and chanted this spell: "Seolfor þræd aþring winstre, aþring yfele, aþring wiþ ealle gode cræfte". As her eyes glowed golden, the flames magically grew higher. By doing this, Morgana bound the pendant to the left hand path. If the necklace was worn, any healing spell cast upon this person would be reversed and magnified tenfold.
Agravaine placed the charm around Uther's neck and when Merlin, disguised as Dragoon the Great, tried to heal the King at Arthur's request, he actually killed him, leading Arthur into believing that he had intended to do so in the first place. Gaius later found the pendant and proved to Merlin that Morgana most likely had something to do with Merlin's failure to heal the king. It is unknown if he threw it away or kept it in his chambers after that event (The Wicked Day).