Fredrik Backman said this quote

They hate people and their stories; they have hated for so long and with such intensity that in the end the darkness enveloped their whole bodies until their shapes were no longer discernible. That is also why they are so difficult to defeat, because they can disappear into walls or into the ground or float up. They're ferocious and bloodthirsty, and if you're bitten by one you don't just die; a far more serious and terrible fate lies in store: you lose your imagination. (talking about shadows)