Kiln

The Cauldron is the Azure Grimoire's original form and it's purpose is to gather souls to smelt the sword in its body.

The 13th Prime Field Device, Nu (v-13), was tempered at the bottom of the Kagutsuchi branch office, near the Sheol Gate. 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 (μ-12) embodied the true Azure Grimoire, and became the perfect Murakumo Unit, Kusnagi.