One of two chaotic beings, Entropy manifested itself in the form of Pazuzu, a demon prince. Through his influence, the Triumvirate was able to spark the War of Humanity in which Chaos was able to gain a foothold in Creation.
Entropy joined its brother, Discord, to challenge Proteus in the Chaos Wars. When Ayar, the Creator, realized that His creations were about to be destroyed, he dispersed Entropy and Discord all about Creation, adding chaos to it. The Plane of Chaos was the area most affected by this dispersal.