Element Arc

The Element Arc is a story arc in Shinyaku Toaru Majutsu no Index, covered in Shinyaku Toaru Majutsu no Index Volume 16.

Following the disappearance of Kamisato Kakeru after Kihara Yuiitsu stole World Rejecter, Academy City has been hit by an abnormal heat wave exceeding 55 degrees, taking out the city's infrastructure. At the same time, the city has also been infested by forms known as Elements.

Elements
In order to draw out the Kamisato Faction, who had disappeared after their initial confrontation, Yuiitsu unleashed the Elements, reduced forms which were a hybrid of science and magic. Created in the shape of animals and plants, with a reverse of the process which converts animal and plant remains into petroleum over time, and animated by a magic-based core corresponding to one of the four elements,  the Elements possessed the ability to.

Heat Wave
After initial contact with the Elements, the Kamisato Faction performed experiments on captured subjects and determined that their activity was lowered in temperatures exceeding 40 degrees Celsius. Following this finding, Fran used her space station Bunny Grey Messenger to flood Academy City in microwaves, creating an artificial heat wave with temperatures exceeding 55 degrees Celsius.