In the world of Angels of Decay there is almost no land left on earth. There are a few isles here and there, but the majority of the continents disappeared under water. Sea life evolved to the point where it became a major threat to mankind. Humans did not have a choice, they had to move up, to the sky. Because of their new habitat, humans evolved. They start growing wings, naturally or via technology. They found a new source of energy: Liquid Light.
Those who were born with wings are among the highly placed people in this new civilization, and they oppressed the ones that do not have the privilege to fly.