The Fall of Aegis

Dream of a place that has almost everything someone would want in it. There is no poverty, pollution, or anything that seems to do harm to the world. Everyone seems happy, content, and follows a life of comfort and joy. Love and hope are almost tangible things, and anyone can feel it while in that place. Birds sing, and the animals there mingle and live in harmony with the humans. The shining sun is a stable object in the sky.
Now, with all of that in mind, flip that image upside down. The aforementioned poverty and pollution come back in full force. The tangible love and hope sours and shrivels into something darker. Car alarms and screams are a part of this place's background noise. Smog clouds cover the previously blue sky.
These two places seem worlds away from each other, but they are only separated by a wall that seems to split the world in half. The wall, giving the impression that it rises up into the heavens and goes beyond even that, is the clean side's salvation.
At the same time, it is the darker side's bane. Many live on either side of the wall, but the darker side's people are dying due to the pollution that the clean side has trapped them in. The clean side, Sanctum, lets its people live a happy life, while the children of the darker side, the Rot, were doomed from birth to short lives until sickness or confinement claimed them.
The wall Aegis has stood for two hundred and ninety-eight years, taken care of by the Sentinels of Sanctum. Guarded by their elite, Sanctum's citizens hardly ever thought about the Rot's blight. After all, their king would always take care of them.
Something out of sight is out of mind, but this blight's salvation is about to be in full view.