The Lord of Nightmares also known as LoN, L-sama or Mother to some Mazoku's. She is considered the most powerful Dark Lord. They mention her several times in the anime, but the only times when they actually show her is in the theme song of Next, which she is shown as a girl with long blonde hair, and at the end of Next which she is shown in Lina's body. It is accepted that she takes the form of a young girl.
LoN is also known to be the creator and the destroyer of everything. She is the Sea of Chaos. The Sea of Chaos is where the 4 staffs are held. Each staff holds a different world on it. LoN also created a god that is evil (a Mazoku) in each, but to balance it she also created a god that is good. (a Shinzoku) These gods are equal in power. In Lina's world LoN created the Holy Dragon Ceiphied and the Mazoku Ruby Eye Shabranigdo to fight. In the world which the anime Lost Universe takes place, LoN created the Holy Dragon Volphied and the Mazoku Dark Star to battle.
The 4 Dark lords that the Lord of Nightmares created are Ruby Eye Shabranigdo, Chaotic Blue, Death Fog, and Dark Star. Each of these Dark Lords exists in a different world. Under the Dark Lord Shabranigdo there are 5 servants. Under each servant, they create more servants to serve them except for the Deep Sea Dolphin.
The two spells, Giga Slave and Ragna Blade, both draw energy from the LoN. That makes them extremely powerful spells, but also dangerous. If the Giga Slave is miscast, it could cause the entire world to become consumed into chaos.

---Map of Dark Lords/Mazoku's---
To the left, there is a Mazoku Chart, including LoN. The very top symbol represents LoN. The symbol under LoN is either Destroyer or nothing at all. Then there are four symbols underneath which represent the for staffs LoN created. The red world is at the bottom of the four, where the Slayers story is held. The world to the right is the black world. It is where the anime, Lost Universe, takes place. There is no information about the other two worlds. The five symbols underneath the red world are the symbols that represent the 5 loyal servants (except one) of Shabrangdo.

(The webmaster's anime nickname is LoN. Just wanted to point that out... ^^)