It came from the “human” named Aleister Crowley.

He lay collapsed on his back, but a thick blade appeared far too suddenly and mercilessly pierced the left side of his chest.

The grimoire library spoke the name of that deadly weapon.

“The Sword of Damocles…?”

Immediately, light exploded from the wound and gravity seemed to vanish for Kamijou’s group.

“Hm, hm, hm, hm, hmmm☆”

Now that everyone was gone, someone appeared as if to view the magician whose heart had been skewered.

[...]

There was not just one person there.

There was a man. There was a woman. There was an adult. There was a child. There was a saint. There was a sinner.

Aleister’s generic appearance had left those who saw him with a variety of impressions, but this very strange scene looked like each of those different faces had been isolated and brought to life.

[...]

“That is because I had always sealed many different possibilities within my single body. But once I released multiple versions of myself into the world, it became apparent they would only clash and I could not expect them to work together. That is why I had to bind myself and contain every version of myself to a single coordinate. So as usual, I did not think it through and regretted my actions.”

The group spoke.

"[...]I would have branched out almost endlessly. This will create every version of me born from the various ‘ifs’.”

Smiles surrounded her.

And they undoubtedly belonged to the “human” named Aleister Crowley.

“If the number calculated out by the former Tree Diagram and proven by the Reading Thoth 78 is accurate, the number of options available to me and the number of branches to which those would lead are 1,083,092,867. In other words, that many Aleister Crowleys exist as those many possibilities. You have witnessed that moment. Yes, no matter what you do now, you have only witnessed it. As if you are watching fireworks being launched from this single spot.”

Toaru Majutsu no Index New Testament vol.18 epilogue