Ursula's Cauldron is where Ursula casts evil spells, creates illusions, and brews potions. The cauldron also enables Ursula Remote-Viewing via Flotsam and Jetsam's glowing yellow eyes to project images onto a crystal ball for Ursula to spy on others.
In the original movie, Ursula uses the crystal bubbles as a spyglass the majority of the time, with Flotsam and Jetsam as the receivers.
During Poor Unfortunate Souls, Ursula is able to create spectral images with just waves of her hands, and manipulate them at will. However, when Ursula wants to create powerful magic, she will use the many ingredients she keeps in her potion cabinet to create these spells, such as when she needs to turn Ariel into a human, or when she needs to become Vanessa.
The cauldron serves the exact same purpose in the TV series, the only difference is that Ursula often plays around with the bubble; bouncing it as if it were a ball.
In the episode, Ariel's Treasures, Ursula is able to transfer magic over through the bubble onto the projected object within. Like when she used the whirlwind spell to possess Ariel's collection.
Later in that episode, Ariel destroyed the crystal bubble.
The Little Mermaid (NES)
During the first phase of the final battle with Ursula she uses the cauldron as her primary form of attack. From it, she summons various sea creatures to attack Ariel. Ariel must throw them back at her to defeat Ursula.
Ariel's Story Studio
During the Poor Unfortunate Souls scene, Ursula's cauldron acts as a minigame of sorts. Flotsam and Jetsam will instruct the player, in riddles, to put three different colored potions into the cauldron. The correct combination will reward the player with scenes from the movie, redubbed by the original actors, with new music as well. The combination will change every occasion, for different scenes.
The scenes featured are:
1) The Dining Room conversation with Eric and Grimsby.
2) The Scuttle Scene
3) King Triton worrying about Ariel.
In the first Kingdom Hearts, the cauldron is seen being used by Ursula to project images of the Sora and his gang. Though unlike in the original movie, the spy images are projected from the inside of the cauldron instead of a bubble on the outside. But like the original movie, she can project these images from out of the cauldron and manipulate them.
King Triton describes the cauldron as the source of Ursula's power, and gives hints telling that the cauldron is her weakness.
The cauldron does not make a physical appearance in the musical. Its function is replaced by a Magic Nautilus Shell which serves as the source of Ursula's powers. However the cauldron still receives its mention in Poor Unfortunate Souls and a deleted story arc featuring Vanessa, had Ursula whipping up a potion to turn herself into a human, so the cauldron may have originally been a part of the Musicals Workshop.
- In Ariel's Story Studio, because Grimsby's actor Ben Wright has passed away, he was replaced with a new actor to redub his conversation with Eric in the dining hall.
- In certain art of Ursula's lair, one can see that the caldron is inside a sunken pit in the middle of the main hall. It was revealed in the deleted scene involving Harold, that Ursula would use this pit as a spa which she would fill with magic cauldron water.