Dendar, the Night Serpent

When the first mortal creature began to dream, Dendar, the Night Serpent, was created. She fed on the fears and nightmares of all mortal creatures and through them, grew larger and larger over time.
First, Dendar, Mother of the Night Parade, gave birth to Deriloz and Jagorroth, the Twin Gods of Destruction. They are sealed away and lay sleeping beneath the surface of Zameen. Then the Serpent Mother also created the snakefolk, the yuan-ti, to serve and devote themselves to her. Deep within the Jungles of Chulweyné, they built great cities and monuments in her honor.
Siding with the other primordials during the Dawn War, Dendar threatened to consume all of Zameen, earning her the title Eater of the World. When Io sundered the planes into separate realms, Dendar found herself stranding in the Abyss, cut off from those on whose nightmares she fed.
Still today cults devoted to the worship of Dendar operate in secret, kidnapping victims and feeding them poison to cause them to dream. Parents all across Polnarus warn their children by telling them the following nursery rhyme:

In Shar's domain of night I rest,

So dreams may show me how I'm bless'd.

If screams of terror break my sleep,

Then Dendar's sunk her fang's too deep.