Kiln

The Cauldron (窯) is the Azure Grimoire's original form. Its purpose is to gather souls to smelt the sword in its body.

The 13th Prime Field Device, Nu-13, was tempered at the bottom of the Kagutsuchi branch office. The Murakumo Unit, in order to be complete, needed the other grimoire, which Ragna the Bloodedge had. When Ragna and Nu fell into the cauldron and were smelted together, something went wrong and resulted in the Black Beast, a failed experiment, being created. Mu-12 embodied the true Azure, and became the perfect Murakumo Unit, Kusanagi.

Similarly to a Nox Nyctores, it takes thousands of souls to create a Murakumo Unit, which explains why the Kagutsuchi NOL branch was completely empty. Hazama had sacrificed the NOL soldiers inside the branch so both Mu-12 and Nu-13 could be smelted. Judging by most endings besides being able to create the perfect Murakumo Unit, the cauldron has the power to send a being into the past by 100 years or more as shown with Nu, Ragna and Jin. It also has the power to send Makoto Nanaya (or anyone with strong attachments to their home or anything from it) into an alternate universe with help from the failed boundary interface/prime field devices.

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. Noel and Bloodedge regained some of their memories by simply looking at the Cauldron. Hakumen and Jubei, while being near the Cauldron were also weakened to the point that it enabled Terumi to overpower both of them at the same time.

Mu-12's Cauldron is located at the top of the NOL branch of Kagutsuchi and resembles a cocoon, while Nu-13's is located at the bottom and has a shape of a sword.