Ursula's Necklace

Ursula's necklace is featured in the Little Mermaid, has it was used to capture Ariel's voice as part of the deal to grant her legs. The movie implies that it may be a magical necklace as it is often glows when Ursula seems to start using a spell such as taking Ariel's voice or even transforming into Vanessa.

Behind the Scenes
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. It was destroyed by Ariel, and took Ursula with it.

The Little Mermaid
Close to the end of the Little Mermaid, Scuttle snaps the necklace of Vanessa's neck. It falls on deck and shatters, giving Ariel back her voice.

The Little Mermaid (musical)
In the Broadway version of the movie, Ariel breaks the necklace which is the source of Ursula's powers.