Boundary

The Boundary is the place of origin. It is a world beyond the Gate of Sheol. The Master Unit, Amaterasu, is in this place, and the only one who can find the Unit is a Successor of the Blue.

Only a few can Observe the multiple timelines in the Boundary, like Takamagahara System and Rachel Alucard. 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. Full exposure, however, is deadly, as the physical body cannot withstand the whole of the knowledge. 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 Litchi's and Hazama's conversation, there is more than one Boundary.

There have been cases where the effect of coming into contact with the Boundary manifested itself.
 * Individuals with amnesia such as Noel Vermillion, Bloodedge, and Kazuma Kval recover their lost memories by standing near a Kiln, which allows them to briefly gaze at the surface of the Boundary.
 * In Arakune's case, stepping into the Boundary transformed him into a black, amorphous creature, and spat him back out, leaving Arakune obsessed with finding the energy of the Blue, and now hunts down those that exude the same energy or traces of the Blue Grimoire.
 * Litchi Faye Ling started having fits of blacking out and witnessing events of other versions of herself, as well as having a desire to obtain the Blue Grimoire. As she draws power from the Boundary, which has since corrupted her (to an unknown extent), it also slowly erodes her mind, and over-usage may end with her turning into something like Arakune.
 * Hakumen traveled through the Boundary at least twice. Once to the past as Jin Kisaragi, resulting in his current transformation to the white knight. The other time was after the Great Dark War, when he had to sacrifice himself in an attempt to seal Yūki Terumi away for eternity. Hakumen drifted inside the Narrow Space (挟問) for roughly 90 years before being salvaged by Kokonoe, while Terumi was released by Takamagahara. In Hakumen's case, his incomplete materialization sipped away his heart and mind, threatening to drag him back into the Boundary.
 * Relius Clover was devoured by the Black Beast and ended up traveling to the future. He lost some memories, but quickly regained them afterwards. During his journey, he discovered the truth about souls and developed an indifferent attitude, looking at living beings as only objects.
 * In Makoto Nanaya's case, only her consciousness was dragged into a different timeline where Noel did not exist. Due to her strong physical and mental state, and with the help of Rachel, she managed to return to the timeline she truly belonged.