In the Slayers universe the Lord of Nightmares is the ultimate being, the creator of at least four parallel worlds. The Claire Bible, an artifact containing the knowledge of a fallen shinzoku, Aqualord Ragradia, states that the Lord of Nightmares seeks to regain its "true form", which is only attainable by destroying these worlds and returning them to the chaos that it itself is. For unexplained reasons, though, the Lord of Nightmares has not acted upon this desire by itself so far.
On each of these worlds are gods (shinzoku, lit. "godly race") and demons (mazoku, lit. "monster race"), fighting without end. Should the gods win the war in a world, that world will be at peace. Should the demons win, the world will be destroyed and returned to the Sea of Chaos. In Lina's world, the supreme god is Flare Dragon Ceiphied, and the supreme demon is the Ruby-Eye Shabranigdo. Long ago, their war ended more or less in a stalemate, when Ceiphied was able to split Shabranigdo's existence into seven pieces in order to prevent him from coming back to life, then sealing them within human souls. As the souls are reincarnated, the individual fragments would therefore be worn down until Shabranigdo himself would be destroyed. However, Ceiphied was so exhausted by this that he himself sank into the Sea of Chaos, leaving four parts of himself in the world.
A millennium before the events in Slayers, one of Ruby-Eye's fragments (which was sealed in the body of Lei Magnus, a very powerful sorcerer) revived and began the Resurrection War (降魔戦争 Kōma-sensō, alternately "War of Demon Conquering") against one of the parts of Ceiphied, the Water Dragon King, also known as Aqualord Ragradia. Ultimately, the piece of Shabranigdo won, but Aqualord, using the last remnants of her power, sealed him into a block of magical ice within the Kataart Mountains. Nevertheless, Shabranigdo's lieutenants remained at liberty, sealing a part of the world within a magical barrier, through which only mazoku could pass.
There are four types of magic within the Slayers universe. Black magic spells, such as the famous Dragon Slave, call directly on the powers of the mazoku and are capable of causing enormous damage. White magic spells are of an obscure origin and are used for healing or protection. Shamanistic magic is focused on manipulation and alteration of the basic elements of the natural world (earth, wind, fire, water and spirit) and contains spell for both offense and convenience, such as Raywing, Fireball, or Elmekia Lance. Holy magic uses the power of the shinzoku, but the aforementioned barrier made its usage impossible before the death of the mazoku Hellmaster Phibrizzo. As a rule, mazoku can only be harmed by spiritual (astral) shamanistic magic, holy magic, or black magic which draws power from another mazoku with greater might than the target.
Above all other magic, however, are the immensely destructive spells drawing power from the Lord of Nightmares. The two spells of this class are the Ragna Blade, capable of cutting through any obstacle or being, and the Giga Slave, which can kill any opponent, but which could also destroy the world itself if the spell is miscast. Some have clamed that this is a fifth form of magic called Chaos magic (after the Lord of Nightmares).
