Guinevere's Enchanted Bracelet was a gift that was given to Guinevere Pendragon by her former love interest Lancelot, who was actually enchanted by Morgana to make Gwen fall back in love with him in order to make Arthur break up with her and help Morgana prevent Gwen from becoming queen.
When Agravaine told Morgana of Arthur's decision to marry Guinevere, Morgana decided to raise Lancelot from the dead so she would break up Arthur and Gwen's relationship so Gwen would not become queen. Her first attempt when Lancelot appeared at Arthur's engagement tournament did not work, so Morgana enchanted a bracelet to make Gwen's old feelings for Lancelot return. Lancelot visited Gwen's house one morning and he presented the bracelet to her as a wedding gift, which she graciously accepted.
While wearing the bracelet, Gwen started to act strange and visited Lancelot in his tent, where she gave him a very intimate show of affection; after that she claimed she did know why she was in his tent in the first place. After Lancelot disqualified himself from the tournament, Lancelot met Gwen in the great hall, where under the enchantment Gwen kissed him. Agravaine and Lancelot had planned for Arthur to see the two kissing in the great hall; Arthur caught them kissing and he attacked and almost killed Lancelot, unknowingly with Merlin's help. Gwen intervened, and Arthur threw her and Lancelot in prison; in her cell, Gwen threw the bracelet away, and since then no one has ever found the enchanted bracelet, although there is a chance that Agravaine took care of it once it had served its purpose (Lancelot Du Lac).