Founded 1,500 years ago, at the end of the War of the Mark, the Twelve is an institute of arcane learning that enjoys the patronage of the dragon marked houses. The Twelve operates out of a thirteen-tiered keep that floats above the city of Korth in Karrnath, pursuing its mission to study dragon marks, their magical and practical applications, and their potential for profit.
The Committee of Twelve oversees the institute. In theory, the committee is made up of one member from each of the dragon marked houses, although its membership has fluctuated over time. At present, House Cannith has three representatives (one for each branch of the house), and the split of House Thuranni from House Phiarlan means that the committee actually numbers fifteen representatives.
An adventurer who has a significant reputation within a dragon marked house might be able to draw on the vast resources of the Twelve. The institute has a tremendous array of mystical supplies, and it maintains vast libraries of arcane learning and rituals. Its members and fellows are experts on nearly every topic imaginable.
Source: Eberron Campaign Guide
At the conclusion of the War of the Mark, Lord Hadran d’Cannith suggested that the houses formally cement their alliance by creating a citadel—a center for research and study of both arcane magic and the potential of the dragonmarked. Though there were only ten dragonmarked houses at the time, the architect and artificer Alder d’Cannith convinced the committee to name the institute the Twelve, based on his belief that there were twelve true dragonmarks in addition to the shattered Mark of Death. Alder was a brilliant man whose works had played a critical role in the War of the Mark, and the members of the committee humored him—though few expected the remaining two marks to appear. (The remaining two marks, Warding and Finding, were not “discovered” until after the creation of the Kingdom of Galifar.)
In the beginning, the Twelve played a critical role in shaping the dragonmarked houses, but as the houses have grown in power and spread across the land, its influence has diminished. Nonetheless, the institute remains one of the premier centers for magical research in Khorvaire. By combining the skills and mystical talents of the different marks, the wizards and artificers of the Twelve have created remarkable items. It took the combined skills of House Orien, House Cannith, and House Kundarak to create the safe-deposit vaults that allow Kundarak customers to deposit goods at one bank and withdraw them across the continent. Airships, the lightning rail, even the warforged—these marvels could not have been accomplished without the spirit of cooperation and discovery found among the Twelve.
Should you rise high in the favor of a dragonmarked house, you might be able to call on the resources of the Twelve. This institute possesses impressive mystical workshops, vast arcane libraries, and the talents of some of the most gifted artificers and wizards in Khorvaire—all of which could prove quite useful to an adventurer.
In theory, the Twelve is a neutral foundation: a resource serving all houses equally. However, some speak of a secret cabal at the heart of the Twelve that seeks a return to the days when the institute shaped the future of Khorvaire. Such a group might hope to overturn the Korth Edicts and see the dragonmarked houses rival Khorvaire’s nations in strength. Only the DM can decide if this secret society exists—and how much power it wields.
Source: Dragonmarked