A Kusanagi (クサナギ) 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 currently the only known Kusanagi.
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. With powers far surpassing a BIPFD, a Kusanagi can not only freely enter the Boundary but even has the potential to destroy a Sankishin. This power was desired for the purpose of destroying the Amaterasu Unit and ending the Looping World.
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. 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 be 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 the fragment 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 have been seen in the Looping World with the repeated fusions of Ragna the Bloodedge and Nu-13.