Tegami Bachi Vol 13

In a land of perpetual night, your heart is your light. Amberground is locked in darkness. A man-made star casts only a dim light over the land. The pitch-black wilderness is infested with Gaichuu-colossal insects with metal exoskeletons. The Gaichuu make travel between the cities of Amberground extremely dangerous. But thankfully the Letter Bees, a brave corps of messengers, risk their lives in order to keep the hearts of Amberground connected.
While Lag and his fellow Bees fight a desperate battle against the Gaichuu Cabernet, the dark secrets of Amberground come to light. What unspeakable things did Garrard, director of the Beehive, see in Amberground's capital? And why has former director Largo Lloyd joined the rebel organization Reverse? The answers lie in the top-secret district called Kagerou.