Surrounding the city of Sunblack on all sides is a dome of refracted light called a Prismatic Veil. A consistent feature of root-leaf entangled cities, Prismatic Veils block almost all access to the territory within, except by underground tunnel or hesper-lined gateway. Anything passing through a Prismatic Veil without one of these protections becomes refracted. People do not typically survive this process.
this is actually key worldbuilding for something i reworked in sunblack's elevator pitch a short while ago -- i needed a way to seal off the city from the outside so that the US couldn't just invade it, which it might try to do given the new premise. kekkai sensen used a big cloud of fog and a giant kraken, but i'm a sun-lover and i wanted there to be chances for bright views looking either out or in -- at least in the literal sense.
