A Kusanagi is the perfect fusion of a Boundary Interface Prime Field Device and a piece of the Azure. They possess amazing power, but the process to create one is dangerous, and a miscalculation will nearly always create a Black Beast.
Mu-12 is the only known Kusanagi.
Information
To make a Kusanagi, a BIPFD has to be smelted with a piece of the Azure in either a Cauldron or a Monolith while using a large amount of souls as a catalyst. The resulting power that a Kusanagi has is far beyond what a BIPFD can achieve. With its power, a Kusanagi can not only enter the Boundary, but even destroy a Sankishin. It was for this purpose that the Kusanagi was desired, as it could kill the Amaterasu Unit and end the Looping World. The Zeroth Prime Field Device was the first to become a Kusanagi after it made successful contact with Amaterasu, and it was its power that inspired scientists to replicate its abilities.[1]
During the Ikaruga Civil War, Hazama tried to merge with a young Mu-12 in an effort to create a Kusanagi. She rejected him, and the fusion never occurred.[2] In January 2200, Hazama took Novus Orbis Librarium soldiers stationed at the 13th Hierarchical City of Kagutsuchi branch and sacrificed their souls to a Monolith. He later manipulated Noel Vermillion into accepting her true state as Mu. Noel, having already inherited a piece of the Azure after awakening to the power of the Eye of the Azure, was able to smelt into the first Kusanagi. Although she never demonstrated her god-slaying powers, Mu could handle multiple powerful opponents at the same time.
It is possible to have an Azure Grimoire replace a fraction of the Azure that is required, but a perfect fusion between the two has yet to be seen. End results with an Azure Grimoire have produced Black Beasts, which was seen in the Looping World with the repeated fusions of Ragna the Bloodedge and Nu-13.[3]