Boundary

The Boundary is the place of origin, and the place where Murakumo Units are smelted. It is a world beyond the Sheol Gate. It's a complex space of infinite information dating back to the very beginning, connecting all dimensions, and it's filled with magic element. The Amaterasu Unit is located in the deepest part, and only the Successor of the Blue can find it.

Only a few can Observe the multiple timelines in the Boundary, like the Takamagahara System. At some point, Rachel Alucard was the Amatsumugi, but she abandoned that duty.

The Boundary can be compared to the parallel universe. It appears as an infinite dark empty space that is illuminated by anything living that enters it. Located inside the Boundary is the Blue, and it is described as being the true power of the Boundary itself.

The Boundary is where most who came into contact received their abilities, which is a side effect of receiving vast amount of information from the said place. Jūbei described falling into the Boundary as a fate worse than death, as it felt like not only losing everything one ever was, but also everything one could ever be; in a sense, it takes away one's past and future. Only those with an exceptionally strong will and spirit can travel through the Boundary and remain intact.

According to a conversation between Litchi and Hazama, there is more than one Boundary.

Hakumen drifted inside the narrow space (挟問) for roughly 90 years before being salvaged by Kokonoe Mercury. His incomplete materialization sipped away his heart and mind, threatening to drag him back into the Boundary.