The Shadow

The Shadow is the Sovereign of Magic and Mayhem. It is the literal shadow of Aureon, gaining sentience and godhood as the price paid by Aureon for his arcane knowledge.
The war between Aureon and his shadow rages in all of us. Aureon teaches that we are stronger together, that we should trust in his laws and it is worth it to suffer for the sake of others. The Shadow states that there is no common good, all that matters is what you need, why should you give to those who only take from you?
The Shadow represents ambition. The Pyrinean Creed asserts that the Shadow will lead you down dark paths and tempt you to hurt others to get what you want, and that the Shadow will devour the souls of those who fall prey to this temptation.
A priest of the Cazhaak Creed would tell you this is exactly the sort of lie that the Sovereigns use to keep people docile and servile. They believe the Shadow will show you the path to be the best you can be, even if that path means you have to step on others to get there. Furthermore, many of those who follow the Cazhaak Creed believe that the Shadow created them.
Where Aureon and the Sovereign Host hoarded their power after ascending to godhood, the Cazhaak Creed states that the Shadow shared it's power with it's children, creating many of the monstrous races of Eberron. The Shadow gave the medusa it's petrifying gaze, the troll it's regeneration, the ogre it's fearsome strength, and the harpy it's voice.
The Shadow also represents secrets, and forbidden knowledge, things that you know you shouldn't try to learn. This covers magical secrets, but it also extends further than that; priests of the Shadow often know things that they have no reasonable way of knowing.
The Shadow is generally depicted as Aureon's shadow, or as a shadowy dragon.

Religions

• Cazhaak Creed