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The Cauldron (窯), formally known as the Gate of Sheol (シェオルの門), sometimes called Hell Spring (黄泉坂) or Gate of Judgment (裁きの門) is a gate to the Boundary that is used to extract seithr from it.

There is a Cauldron under every Hierarchical City, and in all important Novus Orbis Librarium facilities.

The Cauldrons are used to access the Boundary, some characters used them to travel in time or between possibilities, and Nox Nyctores Causality Weapons and Dimensional Boundary Interface Prime Field Devices are smelted beyond them. It is also possible to smelt a Cauldron, but it requires a great number of souls.

A special type of Prime Field Devices, Murakumo Units, are smelted in the Cauldrons. Similarly to a Nox Nyctores, it takes thousands of souls to create a Murakumo Unit, which explains why the Kagutsuchi Librarium branch was completely empty. Hazama had sacrificed the NOL soldiers inside the branch so Mu -No. 12- and Nu -No. 13- could be smelted. Smelting the perfect Prime Field, Kusanagi, requires a Murakumo Unit and an Azure Grimoire, which is a portable Cauldron whose purpose is to gather souls to smelt the sword in its body. The failed merge of Azure and Murakumo results in a Black Beast, but it is also possible to smelt a Beast without using them. According to Relius Clover, the core of the Nox Nyctores is actually a compressed Black Beast.

Nu was smelted in the Cauldron at the bottom of the 13th Hierarchical City of Kagutsuchi. Mu obtained the true Azure, so she did not need an Azure Grimoire to become Kusanagi, and it also allowed her to be smelted without a Cauldron, just in a cocoon, though she still needed something to collect souls, this something was a Monolith.

The 5th Hierarchical City of Ibukido had a Monolith that was connected directly to the Cauldron, but it was destroyed, and now this Cauldron is sealed with the Kushinada's Lynchpin.

Magister's City: Ishana has an artificial Cauldron, which cannot be opened, but can still extract the Boundary's energy.

The Cauldron can also kill somebody by mostly reducing their body to almost nothing by sending them into the Boundary. In fact, only being near the Cauldron itself exposes oneself to various side effects of the Boundary. Noel Vermillion and Bloodedge regained some of their memories by simply looking at the Cauldron. Hakumen and Jūbei, while being near the Cauldron were also weakened to the point that it enabled Yūki Terumi to overpower both of them at the same time.

The Cauldron itself is like a living organism, as it has its own "life" and "pulse". This can be detected by those that wield the power of the Azure, as well as those with the Power of Order. Destroying one is a simple task, but only by utilizing the power of the Azure it is possible to truly kill it. It is possible to restore a killed Cauldron.