The Wraith medallion is a magical necklace used to mark the hands of the Wraith's victims, so it knows whose soul to suck out next. Mr. Gold uses it on Regina when he himself promises not to kill her, but the Wraith is banished to the Enchanted Forest where it marks Prince Phillip, with its medallion ending up in Aurora's possession once her prince's soul is sucked out.
After the Curse
Mr. Gold retrieves the Wraith medallion. ("Broken")
When Mr. Gold promises Belle that he won't kill Regina out of vengeance for her locking away his true love, he heads back to his shop and takes out the Wraith medallion, proceeding to press it against Regina's hand so as to mark her. He then takes the medallion out into the woods and uses it - along with his dagger - to summon the Wraith itself. It attempts to suck out Regina's soul but is stopped by Mary Margaret, David and Emma. They all then use Jefferson's hat to banish it to the Enchanted Forest, and the Wraith takes its medallion with it. It immediately meets Aurora, Mulan and Prince Phillip and marks the latter. Once Phillip's soul is successfully sucked out, Mulan gives the medallion to Aurora for safekeeping. ("Broken")