black bullet
Light novel series written by Kanzaki Shiden and illustrated by Ukai Saki. Also adapted into a manga by Morino Hon, serialized in ASCII Media Works' Dengeki Maoh, and a 2014 anime produced by Kinema Citrus and Orange.
The story involves humanity threatened by a pandemic known as Gastrea since 2021, and thus are forced to confine themselves within barriers known as Monoliths, made out of a special metal, Varanium, which is capable of subduing the Gastrea.