Black bullet, vol. 1 (light novel): those who would be gods, paperback/shiden kanzaki

The World Has Already Ended.

The future--where a terrible battle against a parasitic virus called Gastrea has been fought.

and lost.

Humanity is cornered and lives in despair.

Rentaro and Enju face constant danger in their work as a team of anti-Gastrea specialists known as civil security officers.

As if the daily fight against oblivion werent enough, theyll soon face a threat that could destroy all of Tokyo.