(Basically, a special power fills this city and this girl controls it. That special power has been twisted by binding her mind and that has created this angel! In that case…!!)

“I just have to untie the ‘knot’ in this girl’s head!!”

Someone from the science side may have called it a virus.

[...]

To untie the “knot”, Index chose to use words. Altering a human mind may have sounded like a special thing, but it was no different from reading a book to study. Humans naturally had an open “port” for such things.

Index only had to choose the words that would conform to the “knot” and “untie” it by having Last Order hear them.

And that specifically meant…

“A song,” decided Index. “That will be easier to get across than mere words.

Toaru Majutsu no Index vol.13 ch.10 pt.7