Eberron is a campaign setting for the Dungeons and Dragons role-playing game published by Wizards of the Coast.
In 2002, Wizards of the Coast held a contest to find a new campaign setting. The winning entry came from Keith Baker. The Eberron Campaign Setting was published in 2004.
The Tone of Eberron
Eberron differs from generic D&D mainly in atmosphere. Most D&D game worlds have a faux-medieval feel to them, altered slightly by the addition of magic. Eberron is substantially different. Eberron assumes a world where magic has been present from the very beginning. Therefore many things exist in Eberron that are powered by magic whose technological counterparts did not exist on Earth until the 1800s: trains, telegraphs, airships, etc.
The flavor of Eberron is pulp-noir. Anything that can be found in core D&D can be found in Eberron. It just might look a little different. The emphasis is on dynamic action and and intrigue. Think dinosaur-mounted halfling barbarians roaming the plains and psionic spies meeting in dark alleys. In the same setting. Morality and ethics are a lot grayer than is typical in D&D. Orcs are sometimes paladins. Clerics of good deities may not be so good, after all. Corruption flourishes, and the gods are distant, if they even exist at all.
The Substance of Eberron
Eberron features many new options for both players and Dungeon Masters. There are four new races, a new core character class, the Artificer, based around the creation of magic items, and a whole passel of new prestige classes. For the DM, there are quite a few secret organizations and monsters. Also, there are rules for action points, dragonmarks (innate spell-like abilities that certain humanoid noble houses develop) and eldritch doomsday devices.
The History of Eberron
The name of the world, Eberron, comes from the setting's primary creation myth. The world as it exists now was formed when the three creator dragons, Syberis, Eberron, and Khyber got in a fight, nearly destroying all of creation. To avert catastrophe, they merged with the land: Syberis became the sky, Khyber became the Underdark, and Eberron formed the bridge between them.
Most of Eberron's subsequent history is a mess of invasions from other planes and rising and falling empires. The most important stuff comes near the end.
The campaign begins about two years after a devastating conflict called the Last War. The Last War was a 102-year-long bloody civil war caused by a succession crisis that engulfed most of the continent of Khorvaire. (It was so called because everyone figured that after they were done with this war, no-one would ever want to fight one again.) The Last War shattered the ancient kingdom of Galifar (a large, U.S.-like nation comprising several smaller countries). The Last War resulted in the total destruction of the country Cyre, leaving a fog-shrouded wasteland called the Mournland. The Last War left an indelible mark on the entire world, and its aftermath can serve as the basis for many adventures.
In short, Eberron kicks ass. In my opinion, it's the best thing since sliced goblins.