Dragon Cauldron

Dragon Cauldron is the penultimate book of the Dragon Series by Laurence Yep. It was first published in 1991 and is a direct sequel to the earlier books, Dragon of the Lost Sea and Dragon Steel. After Dragon Cauldron, the events in the Dragon Series are tied up in the final installation, Dragon War.

After escaping King Sambar XII's undersea palace, Shimmer, Thorn, Monkey, Indigo and Civet set out to repair Baldy's Cauldron, after it was damaged in the events of Dragon Steel. The five end up on an enchanted island that disables all magic and is impossible to fly or swim away from. A mysterious dog provides them a means of escaping, but doing this unwittingly unleashes the Nameless One into the world, an ancient and powerful evil.

Tropes used in Dragon Cauldron include:

Big Bad: The Boneless King.
Gag Nose: Shimmer, being a dragon, is rather vain about the length of her snout. When she takes on a human disguise, she gives herself a ridiculously large nose, and because of this, is found out almost immediately.
Hellfire: Living fire, a kind of Lost Technology entombed with the Nameless One. It can burn underwater.
Heroic Sacrifice: Twice Civet gives up her life to allow the others time to flee, and Thorn sacrifices his soul to repair Baldy's Cauldron.
Lost Technology: Living fire.
Non-Human Sidekick: Snowgoose, the Nameless One's huge white dog.
Won't Work On Me: The Boneless King can cast spells that make flight and other magic impossible.
No Waterproofing in the Future: There is a very old tiara set with a pearl that causes the wearer to become possessed by the spirit of a long-dead sorceress. They manage to destroy the pearl by submerging in in wine. Wine is acidic and will dissolve a pearl, but a) not that fast and b) not intensely magical ones, which you'd think would have some kind of protection on them.
Power Source: Civet used up all her magic powers to wreak havoc on the Inland Sea and River Glen. There's a whole Non Sequitur Scene where she puts on a magic tiara and starts spouting prophecies, with the motivation behind it being to recharge her depleted magical powers.
Redemption Equals Death: Civet sacrifices herself to let the others escape, saying that her debt is paid.
Sealed Evil in a Can: The Nameless One, who is sealed in a patch of skin on an enchanted island.
Soul Jar: Baldy's Cauldron. Sort of. It's powered by a soul, and the soul that powers it becomes agelessly immortal. However, it comes at the cost of the person losing their bodily form and becoming the actual cauldron, unable to move, speak, or even control the way their soul is going to be used.
Speak of the Devil: The Nameless One.
Torture Technician: A cheerful and hairy man who believes one should always do one's job well. He's hurt by being called a torturer and prefers the term "facilitator".
The Unfavourite: Thorn slips down to this spot after the incident with the tiara.