The Cauldron is a powerful magical artifact that is said to have been used to create life.
According to the Fae, the Cauldron was where the world was created. All life originated from within it, and it has unimaginable power. It was taken by malevolent beings who used its power for horrible deeds, including forging wicked things within it. It was eventually stolen back and, because it couldn't be destroyed, was hidden and forgotten.
In the series
A Court of Mist and Fury
Rhysand and Feyre travel to meet the Bone Carver at The Prison to seek his advice and bargain with him for any information he can give them regarding the Cauldron. After asking questions of his own, the Bone Carver tells them the story of the Cauldron:
|“|| Long ago, before the High Fae, before man, there was a Cauldron... They say all the magic was contained inside it, that the world was born in it. But it fell into the wrong hands. And great and horrible things were done with it. Things were forged with it. Such wicked things that the Cauldron was eventually stolen back at great cost. It could not be destroyed, for it had Made all things, and if it were broken, then life would cease to be. So it was hidden. And forgotten. Only with that Cauldron could something that is dead be reforged like that.
The Bone Carver reveals that the Cauldron was located at the bottom of a frozen lake in Lapplund, but vanished a long time ago. He says that millennia before Rhysand was born, the Cauldron's three feet were successfully removed from it in an attempt to dull its power. The feet were then hidden at three temples: Cesere, Sangravah and Itica. Judging by the recent attacks on the three temples Rhysand and Feyre deduce that the King of Hybern has stolen the feet and has the Cauldron at full power.
The Bone Carver also states that aside from raising the dead, the Cauldron could also be used to destroy The Wall.
Later on in the book, Feyre and her friends face the King of Hybern in his castle. They discover he has used the Cauldron to resurrect Jurian. As a demonstration to the Mortal Queens, the King uses the Cauldron to forcefully Make Feyre's two sisters, Elain Archeron and Nesta Archeron, into High Fae.
A Court of Wings and Ruin
After Feyre's sister, Nesta, recovered from her shocking magical transformation resulting from her being forcibly dipped into the Cauldron, it was revealed that she had managed to actually steal magical power from the Cauldron during their brief exchange. As such, she can sense the Cauldron's presence, its power, and its intended use in advance.
The Cauldron, under the control of the King of Hybern, is eventually brought with him from Hybern to Prythian via ship to the site where the Final Battle is to be fought. It is situated on a rocky mountain face above the field, near the King's pavilion.
During the Final Battle, the King of Hybern uses the power of the Cauldron to burn entire swathes of soldiers into fine ash within seconds. He first turns this power on the Illyrian army in the sky, destroying an entire regiment that Cassian had been leading. Cassian only barely escaped his death as a result of Nesta calling out to him. Cassian dropped out of the sky at the cry, his loyalty to her safety taking precedence. In doing so, he dodged the Cauldron's burning-light spell entirely. His soldiers, however, were not so lucky. They were turned instantly to ash.
The second attack by the Cauldron was directed on the foot soldiers on the ground, where the Bone Carver was cutting through Hybern's army with his death magic, having agreed to fight for Feyre's cause. The Cauldron's power was unleashed against him, and in a blinding flash of heat and light, it slammed into the Bone Carver, disintegrating him completely in the blink of an eye. Feyre witnessing the attack, was shocked by its raw, destructive power.
Exhausted of its strength and needing time to replenish its power, the Cauldron was out of commission following the second attack. Feyre and Amren, following Stryga the Weaver make their way up the mountain to the King's pavilion, intending to destroy the Cauldron. Stryga is killed by the King of Hybern. Afterwards, Nesta and Cassian fight the King to distract him, so Feyre and Amren to sneak around and get to the Cauldron.
As they approach the Cauldron, Amren reveals her real intention: not to destroy the Cauldron, but to use its power to strip her of her Fae form, so she might take up her real form and be unleashed upon Hybern's forces. Her intended sacrifice, she insists, will end the war quickly in favor of Prythian. Feyre agrees to help Amren and watches as her friend descends beneath the surface of the black water inside the Cauldron.
A moment later, Amren's true form - that of a fiery Angel of Retribution - breaks the surface of the black water and escapes the Cauldron, rising up into the world once more to attack Hybern's forces. She defeats them, but the cost is she burns herself out entirely, until she is no more.
Amren's release breaks the Cauldron, and it begins leaking a type of anti-matter (a black ooze that eats up everything living) into the world. To stop it, Feyre and Rhysand together use their magic to seal the Cauldron's rift. The effort costs Rhysand his life, and nearly does the same to Feyre.
Fortunately, Rhysand is saved by the other High Lords (in a manner similar to how Feyre was saved by them at the end of 'A Court of Thorns and Roses'), and as he rises once more, he tells Feyre that while he was inside the Void of the Cauldron, he felt a familiar presence nearby. He reaches into the Cauldron's black water and draws out a wet, sputtering Amren, who has survived the transition. The Cauldron, she says, transformed her physical body when she shed her otherworldly power. Henceforth, she is Fae (and appears as a female of the High Fae).
After the war, Feyre and Rhysand give the cauldron over to Drakon and Miryam in an attempt to prevent feuding between the High Lords over what happens to the cauldron. Drakon and Miryam agree to bring it back, and keep it hidden on their island that has remained hidden for centuries.
The Cauldron's only weakness is the Book of Breathings, a book made from the last of the molten ore used to forge the Cauldron. The Book contains spells to negate the Cauldron's power, or control it. The book can only be used by that which was reforged, or Made.
The Cauldron can do the following: