Welcome to one of the four worlds that exsisted upon the Sea of Chaos, created by the Lord of Nightmares, L-sama.
The Slayers world is much like medival Europe on Earth, with monarchies ruling the various nations that exsist, and chivalrous knights and soldiers protecting the pesants. However, unlike Earth, magic takes the place of many of the technologies possessed in medival, and modern, times.
The early history of this world is sketchy. It is know up until about 5000 years prior to the Unification, two godlike beings called Flare Dragon Ceiphied and Ruby Eye Shabranigdo were locked in an endless struggle. Ceiphied wished to protect the world, and Shabranigdo desired to return it to the chaos from whence it was born. The battle escalated, and Shabranigdo bore from himself five powerful Mazoku (demon) Lords. Ceiphied, in response, gave birth to the Ryuzoku (Dragons). In the end Ceiphied defeated Shabranigdo and split the evil being's body into seven parts, sealing them away. Exhausted and weak, Ceiphied left four parts of himself to guard the world and sunk back into the Sea of Chaos.
4000 years later, 1000 years before Unification, one of the seven peices of Shabranigdo was revived. The War of the Ressurection of the Demon King saw the Water Dragon Lord, one of the remaining peices of Ceiphied's power, destroyed, though the Demon King was trapped in the north, unable to leave. With their plan to ressurect their master thru the Demon King thwarted, the Five Mazoku Lords, fell silent for many years.
Recent years saw the rise to fame of a young sorceress by the name of Lina Inverse. She has battled and defeated one of the seven pieces of Shabranigdo (The Slayers), destroyed the Mazoku Lords known as Chaos Dragon Gaav and Hellmaster Phibrizo (Slayers NEXT), defeated the Mazoku Lord Deep Sea Dolphin and stopped her from gaining the residual powers of Phibrizo and Gaav (The Slayers IRC RPG — NON-CANON), and most recently before the Unification, saved the world from Dark Star, another Dark God equivalent to the split and sealed Ruby-Eye, who hailed from one of the other worlds on the Sea of Chaos (Slayers TRY).
The defeat of Dark Star left the barriers between the worlds on the Sea of Chaos weak, and extremely susceptable to the shattering of the barriers between universes that was the Unification. Now the people of the Slayers world are just trying to learn to cope with the sudden changes, and make the best of their lives. Heroes like Lina Inverse and her company of adventurers have set off into other iterations to make their names known, and in the case of some people, old memories have come back to haunt them…