The premise in general: humans have to live in walled cities known as Monoliths to shield them from Gastrea. Even then, they aren't safe, as there could be humans inside Monoliths infected with the Gastrea Virus and could easily spread the virus due to its high infection rate, and Stage V Gastrea could destroy a Monolith, breaking humanity's last line of defense.
The Gastrea Virus.
The infectees' skin becomes harder and its muscle strength increases, and once infectees have an erosion rate past 50%, they turn into Eldritch Abominations under transformations that could best be described as pure unadelteratedBody Horror.
Also, it's not only limited to normal humans infected by the virus. Cursed Children, who are born with the virus in their blood, are actually walking time bombs of becoming Eldritch Abominations and they reach the erosion rate in a much slower rate than normal humans. And according to the light novels, there are no effective treatments that will reverse their own erosion rate.
The Fugitive Arc, which is extremely dark for an already dark series. The less said, the better.
The world is so nightmarish that even the mere act of existing is this. First of all, you need to fend for yourself whenever the latest Gastrea that has passed a monolith is roaming around, which will always never end well, and if you're a Cursed Child, you will be viewed as an abomination and will most likely be lynched in a pogrom-style attack. This is before factoring in the constant wars between nations, and the hopeless defensive war against the Gastrea. To top if off, living in this world is a nightmarish existence topped by a nightmarish death.
