“But an unexplainable power cannot be dealt with the same way,” Ollerus spoke slowly and quietly. “The most fearsome thing in this world is to be defeated by an attack that uses an unexplainable power and comes from an incomprehensible place while you have no way of thinking up a countermeasure. The attack is so vague that you can't assign conditions to it, and you don’t even know if there's a direction you can go to avoid it or if you can escape its reach after getting tens of thousands of kilometers away. You now know how horrible that is firsthand.”

[…]

It may have been incomplete, but he had taken two direct hits from Ollerus’s Hliðskjálf. That legendary throne did not actually have any attack functionality, but that was exactly why Ollerus’s spell had evolved to something unexplainable when he forcibly used it as one. Hliðskjálf’s attack was a vague thing that didn't have a defined range or force, so #7 had most likely already lost consciousness from being on the receiving end of it.

Toaru Majutsu no Index SS vol.2 ch.21