Their history and nature.

As polar opposites to the shinzoku, the mazoku are literally a race of evil. While many of them take on humanoid appearances in the physical plane, they actually exist as fully spiritual beings on the astral plane. Thus, physical attacks do no harm to mazoku. Only spells that affect the astral plane or spiritually powered weapons (like the five Dark Star weapons) can do any damage to mazoku, making them formidable opponents indeed.

All mazoku typically desire to bring an end to the world, returning it and themselves to the Sea of Chaos. The Sea of Chaos is another name for the Lord of Nightmares, the ultimate diety across all worlds in the Slayers universe, the one responsible for the creation of the shinzoku and mazoku. As neither a shinzoku nor mazoku herself, she is a power beyond all others in the universe. The shinzoku work to prevent this from happening.

The Lord of Nightmares created four separate worlds that are each governed by a reigning shinzoku and mazoku. These two incredibly powerful entities fight each other for eternity. In the Slayers world, the ruling mazoku is Ruby Eye Shabranigdu. He created five powerful subordinates to aid him in his battles: Hellmaster Phibrizo, Greater Beast Zellas Metallium, Dynast Grausherra, Deep Sea Dolphin and Chaos Dragon Garv.

In eons past, Ruby Eye fought his nemesis, Flare Dragon Ceipheed. The war raged on for untold centuries until Ruby Eye managed to deliver a final blow to Ceipheed. With the last of his power, Ceipheed split his remaining power into four subordinates of his own. He also managed to split Ruby Eye into seven pieces, sealing them away into seven different humans with the hopes that each time one of the pieces was reborn, Ruby Eye would slowly vanish.

With Ruby Eye sealed away, his five subordinates worked to figure out how to resurrect him. Eventually, one thousand years before the beginning of Slayers, the five found a piece of Ruby Eye inside a powerful sorcerer known as Lei Magnus. A massive war erupted between mazoku and ryuzoku: the Kouma War. The five devised a clever plan: four of them would scatter to the four directions and erect a powerful magical barrier that would prevent any aid from entering or leaving. The fifth, Garv, would battle the Aqualord within the battle and resurrect Lei Magnus.

Unfortunately, though Garv was successful in defeating the Aqualord, a repeat of Ceipheed's battle with Ruby Eye happened when Aqualord sealed Lei Magnus in a frozen block of ice and sealed Garv into a human body. Unable to leave their posts, the rest of the mazoku left Garv to his fate and Garv, now partially human, realised he no longer wanted to destroy the world. He wanted to live! He began to work against the mazoku in secret, looking for powerful allies in which to recruit for his cause. During this time, he ran into Val Agares and recruited him as his new retainer: Valgarv.