Boundary

The Boundary (境界) is the place where the various timelines intersect. It seems to be where things go after the events have already happened. Only a few can observe the multiple timelines in the Boundary, like Takamagahara and Rachel Alucard. It appears as an infinite dark empty space that is illuminated by anything living that enters it.

Information
Located inside the Boundary is the Master Unit, Amaterasu, the source of distortions in the timeline, which is why Takamagahara wanted Yūki Terumi to destroy it, as Amaterasu was in Takahamagara's way of them gaining complete control over destiny. Only the "Eye of the Azure" can find Amaterasu in the Boundary.

Also located inside the Boundary is the Azure, 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 exceptional strong will and spirit can travel through the Boundary and remain intact.

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 Cauldron, 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 spit him back out, leaving Arakune obsessed with finding the energy of the Azure, and now hunts down those that exude the same energy or traces of the Azure 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 Azure 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 Dark War, when he had to sacrifice himself in an attempt to seal Yūki Terumi away for eternity. Hakumen drifted inside the Edge (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.

The Boundary is also where Nu -No. 13-'s soul was salvaged, and later implanted onto Lambda -No. 11-.

According to Litchi's and Hazama's conversation, there is more than one Boundary.