Swirl of the Root

Akasha, the Root (根源), is a metaphysical location within the Nasuverse as the "force" that exists at the top of all theories on dimensions. It is the location of the Akashic Records, the Swirl of the Root (根源の渦), which is the source of all events and phenomena in the universe. Existing outside of time, it stores and archives information of all possibilities and events, past, present, and future, of the world. It is the place from where all souls, including those of Heroic Spirits recorded on the Throne of Heroes, originate from and to where they return after death.

In Kara no Kyoukai, "Akashic Records" is a name used to pronounce 「　」 (quoted empty space, equivalent of " "), which is also referred to as "void" (空) that is part of the title, a Japanese word for the Buddhist concept of emptiness, Sunyata. Although, because the Akashic Records is a name, it automatically can't be said to be the same anymore, since 「　」 is just emptiness that can't have a name. Shiki's eyes are presumably linked to the root which gains her the ability to see the lines of mortality. Souren Araya abducted her in the chapter Paradox Spiral and attempted to use her to reach the Root.

Reaching the Root
The ultimate goal of the Magi is to reach it and many focus their experiments into discovering a path to it. The First Magic and Fifth Magic were the result of reaching the path without entering, while the Second, Third, and Fourth Magics were meant as a means to reach it. There are various methods used in attempts to reach it, usually involving reaching the pinnacle of a certain concept. There are also methods meant to reach it through manipulating certain systems, such as the Holy Grail War using Heroic Spirits returning to the Grail as a way of making a hole to it. Once a route is used, it is closed for good upon becoming established. If one isn't entered, it can be left until someone does claim it, thus closing it off for good.

Souren Araya attempted to reach it by tracing the roots of the soul, something that is there but not there, in order to reach a group consciousness. Touko Aozaki studied the human body while aiming to recreate the original human body, under the belief that modern humans could no longer reach Akasha after becoming too diversified genetically. Norikata Emiya wished to accelerate time to observe the end of the universe, believing that it would allow him to reach the Root. Nagato Tohsaka, due to his martial arts background, believed that he could reach Akasha by attaining a state of nothingness and enlightenment. As it has no connection to this actual world, the Church, which is only interested in this world, finds it completely meaningless.

Those who have touched it directly have never returned to the world. Those who have managed to touch it ceased to exist on the spot, as their human souls are either are going back to "where they came from", becoming absorbed into the Root, or something similar. The exact details are unknown, so even magi who have left their names in history have refrained from touching it. There are only the Magicians who have come close, but backed away before touching it like Aoko Aozaki and magi who get close to stabilize their own Magecraft in a hurry. Aoko believes that it is possible those who end up there "become God" in a sense, so there is no reason for them to come back. It is also possible they cannot come back because "being God" isn't truly such a wondrous event as humans may think. Both those who succeeded and failed touched it and couldn't return to the world. It may be they told people about it is because the first person to become God wanted companions out of loneliness, or that their work on that side was so massive that they needed other companions.

Parallel worlds
There are an unlimited number of parallel universes, accessible to others only through the Second Magic. The stories of Kara no Kyoukai and Tsukihime were originally considered both to be small happenings in a single world. Afterward, due to slight differences in the ages of certain people and differences in the makeup of certain families, they can be thought of as subtly shifted parallel worlds. The only place where they are allowed to interact is Ahnenerbe. The world of Fate/Extra shares the same events at that of Fate/stay night up until a certain point. The world of Tsuki no Sango is a world in which the events of Tsukihime didn't take place. The world of Notes takes place in an undetermined future where the world dies while humans continue to live on its body, and it current state involves Crimson Moon.