The Decay is a Core Concept to the Paragon Worlds.

Decay is the power source and infection of the Entity known as Entropy. This energy directly affects the fabric of reality.

There exists an infinite number of parallel worlds in existence, separated by a nearly impenetrable barriers that keep them apart. The Decay threatens to destroy that barrier. In its efforts, the worlds within these barriers are tainted. The core concepts of reality are then twisted, resulting in a deviation of how history moves forward.

The when and how much of Decay in each world is what makes them so dramatically different. Without the Decay, these worlds would all move forward, only very minutely different from one another. With Decay, not only does history unfold differently, but the people are dramatically different in ability.

Some worlds are infected with more Decay than others. While affected different things take place. Most predominately the taint will bring some sort of mystical or change to the laws of reality. Each world page describes how Decay changed the course of reality. The more Decayed, the closer the world is to tearing itself apart. Order and Chaos have some control over that as neither of them want to see worlds crash.