Also known as L-Sama, the Lord of Nightmares is the living embodiment of chaos. She is neither good nor evil. She is the creator of the universe. She has long golden hair. She is sometimes depicted in human form as a young woman wearing a wicked cool black and red leather outfit and wielding a scythe. She resides in the sea of chaos between the four worlds she has created. In the first Slayers series, Lina summoned her and used her power to destroy Shabranigdo. In Slayers Next, Lina called upon her power to kill Hellmaster Phibrizzo.

Ruby-eyed Shabranigdo is the dark lord who oversees Lina's world. 1000 years ago he was resurrected by Hellmaster Phibrizzo. A war with the god Cepheid ensued. This ended with Shabranigdo being split into 7 parts, each sealed within members of the human race. This took all of Cepheid's strength, and the god has remained dormant since the battle. In the first Slayers series, the piece of Shabranigdo within Rezo the Red Priest was reborn and then destroyed.

Dark Star is the dark lord of another world. In Slayers Try, Valgarv tries to resurrect him in Lina's world.

Death Fog and Chaotic Blue are dark lords on the same level as Ruby Eye and Dark Star. Nothing else is known about them.

Zellas is one of the five Mazoku lords who serve Shabranigdo. Zellas makes her home at Wolfpack Island. Unlike the other Mazoku lords, who have both a priest and a general, her only servant is Xellos. Her Mazoku form is a huge werewolf-like monster. Her human form is a platinum-haired woman who wears a sleeveless white dress and bracelets around her arms and ankles. She is a chain smoker.

Xellos is the priest and general of Beastmaster Zellas Metallium. In Next and Try, he tags along with the Slayers and generally makes a nuisance of himself. During the war following Shabranigdo's resurrection 1000 years ago, he single-handedly wiped out a large percentage of the population of Gold Dragons. His human form is familiar to all Slayers fans. His Mazoku form can be seen in the last episode of Slayers Next. A black conical shape materializes, and then Xellos appears in its place. He possesses four magical talismans given to him by Lei Magnus that allow him to cast insanely powerful spells like Blast Bomb. ^_^

Gaav the Demon Dragon King meets his demise in Slayers Next at the hands of Hellmaster Phibrizzo. His human form is an ugly red haired dude in an orange trenchcoat. His Mazoku form is a giant red dragon with three heads. 1000 years ago, in the war that followed the rebirth of Shabranigdo, Shabranigdo channeled a powerful magical attack through Gaav that killed both him and the Water Dragon King. Gaav was cursed by the Water Dragon King to be reborn in human form again and again and again. As his soul eventually became more and more human, his bond of loyalty to Shabranigdo became weak enough that he rebelled against Shabby and attempted to take over the monster race. His plans did not work out as well as he expected.

Raltark is the priest of Gaav, and Rashart is his general. They appear in the novels, but not in the anime.

Not much is known about Deep Sea Dolphin. To my knowledge, she does not appear in any of the 3 Slayers series. She is rumored to be insane as the result of something Xellos or Zellas did. Her human form is rumored to be a blue-haired woman. You can make an educated guess about her Mazoku form based on her name. Nothing is known about her servants. Her domain is Demon Ocean.

Dynast does not appear in the series, only in the novels. He resides at the North Pole. One of the forms he takes is an imposing figure in silver armor with horns on the helmet. I've also heard that his human form is a young man (about Zelgadis's age) with long black hair who wears a black coat. His Mazoku form is an expansive black shadow. He has a priest named Grau and a general named Sherra. I've also heard that he has another general named Norst and another priest named Grou. He is rumored to have a cold and dignified demeanor, and is the most intelligent and ruthless of the Mazoku lords.

Grau is Dynast's priest, whom I know absolutely nothing about.

Like the ill-fated Chaos Dragon Gaav, Hellmaster Phibrizzo was also destroyed in Slayers Next. He was the most powerful of Shabranigdo's minions, and was responsible for his resurrection 1000 years ago. In the war that followed, all of Phibrizzo's servants were destroyed. In Next, he 'borrowed' Xellos from Beastmaster Zellas Metallium and used him to manipulate Lina. His home in Lina's world was the Desert of Destruction. Phibrizzo's Mazoku form is a black shape with jagged edges and glowing white patches that resembles a Rorschak inkblot. It can be seen briefly in the last episode of Slayers Next. His human form was a cute little kid. Devious, very devious indeed.