Black World

Black World is an extra-dimensional world of nothingness. In the Shinyaku Toaru Majutsu no Index Light Novel Volume 09, it is where most of the battles between Kamijou Touma and Othinus took place.

Principles
It is an empty world where everything is black. 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.

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.