Ursula's Cauldron

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.

The Original Movie
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 Series
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 occassion, 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.

Kingdom Hearts
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.

King Triton describes the cauldron as the source of Ursula's power, and gives hints telling that the cauldron is her weakness.

Then, during the boss fight with Ursula, she will throw elemental potions into the cauldron, which Sora must counter with spells on his own. Should Ursula manage to get enough potions of her own in the cauldron, the party will be hit with a devastating attack. However, should Sora manage to put enough of his spells into the cauldron, the magic will backfire on Ursula, stunning her, and giving Sora the opportunity to attack.

The Cauldron and the Lair are never seen in the second game.

Trivia

 * 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.