The Sea Witch Necklaces are a piece of enchanted jewelry that serve the purpose of the user, which have multiple uses that typically aid poor intentions. All characters from the cecaelia family are present wearing one, which all have similar appearances of a golden seashell.
They are used to enhance the user's abilities, for instance, Ursula is well-known for using hers to store Ariel's voice after she had signed a contract in exchange for human legs. Sometime after becoming Captain, Uma had her team of pirates track down the pieces on the Isle of the Lost to have it repaired after it was shattered in The Little Mermaid by Scuttle.
Background Information[]
Item origins[]

The necklace as seen in The Little Mermaid worn by Ursula as Vanessa.
Further information: The Little Mermaid
When Princess Ariel visited Ursula, the latter offered to make a deal with the young mermaid that resulted in Ariel losing her voice in order to become human; Ursula kept her voice stored within the necklace, wanting to prevent Ariel from becoming permanently human. Ursula used Ariel's voice as a last effort by turning herself into a woman named Vanessa where she used the stored voice to hypnotize Prince Eric who fell under its spell - believing that Vanessa had been the one to have rescued him after recognising the voice. This is the first time we are introduced to this type of necklace.
History[]
Rise of the Isle of the Lost: A Descendants Novel[]
Wanting it repaired, Uma, Ursula's daughter and Captain of the Lost Revenge pirate crew, had her team track down the shattered pieces that were on the Isle of the Lost. Once fixed, Uma then uses it to track down the trident of King Triton.
Descendants 2[]
After capturing Ben, Uma admitted to the King that she had plans to use it. After appearing at Cotillion, she then used it after jumping into the sea where she then grew in size and gained tentacles.
Under The Sea: A Descendants Story[]
When Mal discovered Dizzy Tremaine wearing the necklace, Uma began talking through Dizzy as a vessel to confront her rival before Gil and Harry appeared, Uma then used its power to once again talk through all three.
Descendants 3[]
After Mal retrieved her father's ember, Uma took it from her claiming that she would be best off keeping it safe before storing it inside her necklace. After Audrey trapped Mal and her friends inside Fairy Cottage, Uma and Mal combined the powers of both the necklace and Ember that allowed them to free all of them who were trapped inside, during Mal's battle with Audrey, Uma returned to her side after turning her back on Mal after the latter lied.[1]Uma then used the power of the necklace once again to combine its magic with the Ember that resulted in stopping Audrey.
Descendants: The Rise of Red[]
Uma continued to wear it after her promotion as Principal of Auradon Prep though never used its magic.
During her time at Merlin Academy, Uliana gained her own necklace that she wore around her neck with it having the power to summon her lair that lived below the Black Lagoon.