The Dreaming Dark

In the spaces between sleeping and waking, nightmares wait with grasping claws. The quori spirits that rule Dal Quor spent a thousand years seizing control of the continent of Sarlona. Now they have turned their many eyes toward Khorvaire. The ultimate goal of the Dreaming Dark is to control all the nations of the world. The plane of Dal Quor goes through cycles of light and darkness, and the current age is coming to an end. The nightmare spirits that rule the plane believe that by seizing control of mortal civilization—by forcing all the people of Eberron to dream the dreams they devise—they can maintain this current age of darkness indefinitely. So the fiends of the Dreaming Dark aren't merely hungry for power; they are fighting for survival.
The quori can't manifest physically in Khorvaire, but they have other options. They can possess willing humanoids, creating agents known as the Inspired. They can also manipulate the dreams of mortals. The Dreaming Dark can trick a priest by sending dreams that appear to be divine visions, or inspire a soldier to rebel against their lord.
The greatest weapon of the Dreaming Dark is its ability to infiltrate other organizations. Anyone could become an Inspired vessel, or be prompted into taking foolish action by a dream. Its agents are scattered across the world and have no obvious connection to one another, until they reveal their true nature and purpose.