Toaru Majutsu no Index [New Testament] (by: Kazuma Kamachi) N

THIS IS NOT A FANFIC BUT THE ORIGINAL
Watch the anime until Season 3 then start reading this.

Translated by : Baka-Tsuki.org

Written by: Kazuma Kamachi

Posted here in Webnovel for easier reading experience.
Note: no need to give power stone, just support the original translator and the author if you have the chance.