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The monsters in Slayers aren't like your typical cute, friendly monsters in Pokemon andDigimon. The monsters in Slayers are in fact, the total opposite. Monsters are extremely powerful, and normal forces cannot defeat them. Their main goal is to plunge the world into darkness and destruction. And they don't care who they have to kill in order to achieve this goal. Monsters can move freely between worlds, and they can even disguise themselves as humans. There are many monsters that walk among humans, and these monsters are usually agents of another monster, who's under the rule of another monster(the high ranking monsters are usually called "dark lords.") Only certain powerful magic spells can defeat monsters.
The War of the Monsters Fall: The War of the Monsters Fall was huge war between the forces of the gods and the monsters fought thousands of years ago. Both sides suffered heavily, and there was no real victor. The monsters didn't succeed in taking over the world, but the gods couldn't entirely defeat them either. Ever since the war, agents of the monsters and servants of the gods continued to constantly fight and oppose each other.
The Pledge It may be hard to believe but humans and monsters have been known to ally with each other, but it's usually for their own ambitious or evil intentions. One way they've been known join forces is through a pledge called "the pledge of immortality." If a human makes the pledge with a monster, the two have a contract: the monster will give the human the power of immortality if the human promises to become allied with the monster race. The only way to break this contract is to kill the monster or destroy the pledge stone(it's not as easy as it sounds, though!)
Shabranigdo is the head monster of the real world(the world where humans live.) So of course he's one of the most powerful. He also created five other powerful monsters to serve under him. The two that have a strong impact in the Slayers series are:
Gaav the Demon Dragon King
and Hellmaster Phibrizzo
Dark Star is like an equivalent to Shabranigdo, except that he exists in the overworld(another world.) Like Shabranigdo, he's an extremely powerful dark lord of destruction.
The gods are opposites of the monsters...they want nothing but peace in the world. Sephied is the main god of the real world, and Volphied is the same for the overworld(I think they were both defeated in The War of the Monsters Fall.) There are servants of the gods in the overworld, who only want to protect that world. Dragons, who are also servants of the gods, try to keep the peace in the real world. There are lots of different kinds of dragons(the top dragons are the Fire Dragon King and the Water Dragon King; both were defeated in The War of the Monsters Fall.) Dragons have their own kinds of magic and holy spells(usually in another language.) Their main attack is to shoot "laser breath" out of their mouths. Like monsters, they have the ability to take a human form if they want. The highest ranking and most powerful dragons are the golden dragons.
Ancient Dragon Genocide: Long ago, the golden dragons massacred the ancient dragons because they feared their growing power; they feared that the ancient dragons might bump them off from the top of the dragon rank. The golden dragons tried to cover this up by saying that the ancient dragons were out of control with power and had to be terminated. But the truth eventually leaked out(see the Slayers Try series for more info!)
Sirius, an overworlder
Golden dragons shooting off laser breath
"Theirs is an existence too large to comprehend. However, I will tell you what can be told: The mother of all darkness. The true lord of the monster race. One who longs to regain their original form. A darkness beyond blackest pitch. Deeper than the deepest night. One who shines like gold upon the sea of chaos. A void within everything. The source of all chaos. One who has dominion over all terrible dreams. In short, the Lord of Nightmares."