Ursula's Necklace

Ursula's Necklace is an enchanted objected featured in the Little Mermaid and various other media.

The Little Mermaid
In the original movie, the necklace is used to hold Ariel 's voice when she sells it to Ursula in exchange for legs.

During Poor Unfortunate Souls Ursula asks for the voice, in an attempt to make sure Eric won't be able to recognize her, to which Ariel agrees, and signs a contract. Ursula commands Ariel to sing, as she summons up a pair of hands from her magic cauldron, which slowly approach her as she sings, and her voice pulsates wildly in her throat. The hands then rip Ariel's voice from her throat, still singing, and proceed to place it inside the necklace, silencing it.

The necklace is then used later in the movie, during the night of the second day, to hypnotize Prince Eric into marrying Ursula, disguised as the maiden Vanessa . She approached him on the beach singing, as the necklace glowed golden, and bits of Ariel's voice enter Eric's eyes, keeping him under Ursula's control.

During the wedding of the third day, Vanessa's plans go awry when Scuttle and his army of animal friends stormed the wedding ship. As Vanessa is soaking wet from dolphin spit, Scuttle approaches her and snatches the necklace from her, all the while Vanessa chokes him. The two are locked in a stalemate, until Eric's dog Max bites Vanessa's posterior and she is unable to grip Scuttle anymore. The necklace snaps off her neck, breaks on the deck, and Ariel's voice is set free.

The Broadway Musical
Although her necklace does not actually appear in the Broadway version of The Little Mermaid, a similar item known as the Nautilus was a major plotpoint in it, and it was mentioned that Ursula inherited the Nautilus from her father Poseidon on his deathbed, and with it granted enough power to rule half the sea. It was destroyed by Ariel, and took Ursula with it.

It's power versus the Trident was never stated, but Ursula exclaimed that she would use it to store items, and seeing as how its destruction killed Ursula, it's assumed that it was what gave Ursula all her powers.

Kingdom Hearts 2
In Kingdom Hearts II, the necklace serves the same purpose as it did in the movie. Ariel gives her voice to Ursula in exchange for legs, and Ursula then uses the voice to hypnotize Eric as Vanessa.

In the end, it's the necklace which gives away Vanessa's identity, and in retaliation Sora uses his keyblade to destroy it.

Disney Princess Magazine
In an issue of Disney Princess magazine, while the necklace is not directly used, several seashells are used to perform a similar feature in capturing the voices of Ariel and her sisters. In capturing the voices, Ursula states that her desire is to have every sweet voice in the sea so she doesn't have to hear the music.

Ariel manages to defeat Ursula in the end and gives her sisters the shells, which returns their voices.

This issue also implies that all shells can do this, as King Triton stated, "The sound of the sea is captured in every single shell."

Trivia

 * In the original book, the mermaid has her tongue cut off in exchange for legs. In the movie this was changed to having her voice represented by a glowing orb.
 * In the original ending of the movie, the necklace isn't shattered until after Ursula is defeated.
 * In the comic adaptation, Vanessa is singing in her cabinet without the necklace, implying that she can still use Ariel's voice as long as the necklace is intact.
 * If one were to analyze the terms of Ariel's deal, one could argue that, even if Ariel had managed to kiss Eric before sunset, Ursula still could have claimed her through a loophole in the contract. Since Ariel's voice still technically belonged to her, she could have taken Ariel with it as a container.