A New York Times best-selling sci-fi fantasy series with heart.

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.

As Bees and ex-Bees alike gather in Yuusari, the rampage of the unstoppable Gaichuu Cabernet continues—and heart attacks only seem to make it more powerful. As it absorbs the attacks, Cabernet keeps morphing into stranger and more sinister forms. Meanwhile, a tragic accident triggers a different kind of transformation in Niche…