Merlin's gauntlet is a magical item with the ability to lead one to another's greatest weakness. Guinevere uses it to find the missing half of Excalibur for Arthur, but later trades it to Rumplestiltskin where it becomes a target of the Queens of Darkness. Making the mistake of stealing it back from the three who he traded it to, the Dark One finds it to be his undoing when Belle uses it to defeat him.
Before the Curse
Guinevere trades Merlin's gauntlet. ("The Broken Kingdom")
Wanting to put an end to King Arthur's quest for the missing half of Excalibur, Guinevere steals Merlin's gauntlet and she and Lancelot set out to find it themselves. The gauntlet (which leads to one's greatest weakness) guides them to the Vault of the Dark One and then through a door in a Jungle Dimension. They see the missing half - the Dark One's dagger - is guarded, and Rumplestiltskin soon appears. He won't give them the dagger, but instead offers them Sands of Avalon, which can make Excalibur appear whole. Guinevere accepts this offer, and trades the gauntlet. ("The Broken Kingdom") Rumple later returns home with the gauntlet and has Belle clean it for him, but it soon becomes a target of the Queens of Darkness, who kidnap Belle for ransom. Rumple is ordered to bring the gauntlet to Demon's Bluff, which he does, and he ends up giving in in order to save Belle. However, he later returns and steals it back. ("Heroes and Villains")
After the Second Curse
Belle finds something she shouldn't have. ("Heroes and Villains")
Following the deactivation of the Spell of Shattered Sight and the Snow Queen's defeat, Rumplestiltskin has Belle pack her bags and get ready for their real honeymoon outside of Storybrooke and to the city of New York. As she packs, Henry stops by the pawn shop and helps her look for another suitcase. When pulling one down, the boy accidentally knocks a bunch of other items off the shelf, startling Belle. As they go to pick everything up, she discovers the gauntlet which she thought Rumple traded for her long ago. After Henry leaves, Belle does some thinking, and decides to test out the gauntlet to see what her husband's greatest weakness is. Instead of leading her to herself, it lead her to the real dagger, as she has been keeping a fake since they got married. Belle is able to stop Rumple just as he is about to crush Hook's heart and cleave himself from the weapon once and for all. Having obtained control of her husband, she commands him to free Emma and Mary Margaret, who he has frozen, and then transport them to the town line so they can talk. She confronts him about always choosing power over her and how she's not his true love, but his power is. Still feeling very hurt and betrayed, Belle uses the dagger and commands Rumple to leave Storybrooke, thus making it so he cannot get back in or use any of his magic. ("Heroes and Villains")