Aiyoku no Eustia

After the surface of the world succumbed to chaos centuries ago, the city of Novus Aether is kept afloat in the sky solely by the continual prayers offered by the Saint. Years ago, when the Saint's prayer was interrupted, an incident known as the "Gran Forte" occurred, causing a portion of the land to fall and split the lower city into two, separated by a cliff. Countless lives and property were lost or irreversibly affected in what would be known as the greatest disaster in the city's history.

In time, the lowest layer, known as the "Prison", became the home to people who lost much of their former lives during the "Gran Forte". Crime, famine, and disease run rampant, with "The Noncorroding Gold Chains", the ruling organization in the area specializing in the sex trade, keeping what little order that still exists. Meanwhile, a contagious disease which causes the infected to grow wings has spread throughout the city, and a unit commonly known as the "Wing Hunters" has been formed to deal with these people swiftly and often violently.

In this "Prison" resides Caim Astraea, a freelance ex-assassin who does odd jobs for his friend, the head of "The Noncorroding Gold Chains", for money. During one request, Caim finds Eustia, a winged girl who emits the pale purple light characteristic of the "Gran Forte" incident years ago. His encounter with Eustia will take him on a journey which will involve him with the most influential bodies of the city, including the Church and the royal government. As he searches for the meaning to his cruel, difficult life, he will discover the deepest and darkest secrets of Novus Aether...

(Source: Wikipedia)