A Certain Magical Index NT, Vol. 1 (light novel) by Kazuma Kamachi

A short-lived respite...After bringing an end to World War III, Touma Kamijou disappears from the world. Both sides—magic and science—take time to rest and regroup. Accelerator distances himself from his troubled past and adjusts to living an ordinary life with Last Order and Misaka Worst. Meanwhile, Shiage Hamazura uses the secrets he gained in Russia as a bargaining chip and returns to Academy City to re-form Item. It seems like peace is here at last. That is, until a new group called the Freshmen bursts on the scene, forcing Accelerator and Hamazura back into contact with Academy City’s underworld…