The Lord of Nightmares is the greatest power in the Slayers universe, a popular anime in Japan and the US. She is the source of all chaos, who powers the Ragna Blade and the Giga Slave. She lives in the Sea of Chaos, a large black sphere that obliterates all that enters within. Her nickname is "The Golden Lord".
According to Xellos, a Mazoku, the Lord of Nightmares is capricious. She rules over the Dragons and the Mazoku as a Creator God, though she seems to have very little care to influence the world directly. She is pretty aloof from the mortals in the Slayers universe, except for Lina Inverse.
Lina Inverse is the only human who can use the power of the Ragna Blade and the Giga Slave, making her the closest connection to the Lord of Nightmares. At the end of Slayers NEXT, she lost control of the Giga Slave, and the Lord of Nightmares possessed her body. She nearly destroyed the world, but Gourry's love for Lina convinced the Lord of Nightmares to change her mind, and she released Lina's body and spared the world.
All in all, though, the Golden Lord is very mysterious, and not much is known about her except that she is the supreme ruler of the four worlds in the Slayers multiverse.