Black World

Black World is a Phase of the world or reality. As it's name states it is a world of nothingness, and serves as a temporary blank canvas for Othinus after "destroying" the world. In the 9th Shinyaku Toaru Majutsu no Index Light Novel Volume, it is where most of the battles between Kamijou Touma and Othinus took place.

Principles
It is an empty world where everything is black. Somehow the ground is perfectly flat and Touma described it as even more level and free of defect than the silicon wafer of a semiconductor. As it extends through the "horizon", the ground did not rise or fall by even a micron. Also, it is said that everything is pure black that both the sky or the ground can't be distinguished from each other. There is nothing in it, not even valleys, mountains, the sun or the moon. It seems to go on to infinity that when someone continued walking through it, that person won't remember where he is going or came from. Moreover, it can somehow support living beings as if it had air, since both Touma and Othinus had no problem being in there despite the nothingness. However it is likely that with her tremendous power, Othinus is the one that allows this effect to take place.

Shifting and Fluctuating World
Touma initially woke up in this place finding Othinus also here. At first, he still won't accept the reality that the world has truly ended but after walking for a long time through this world to find if there are other people left, he realized that they were the only ones left and that the world has already ended and went back to Othinus determined to bring everything back to normal.

In the final battle of Touma and Othinus, she used the entire black world as her crossbow.