The Dragonmarked Houses are thirteen extended families which control most business throughout Khorvaire. Within the houses, only a small percentage of every generation manifest dragonmarks, which are marks on the body that grant spell-like abilities to those born with them. There used to be thirteen dragonmarks but only twelve remain. Each family possesses only one type of mark, and only one or two races can manifest a particular type of mark. Additionally, only races listed in the Player’s Handbook can manifest a dragonmark at all. With the exception of House Phiarlan and House Thuranni (which both possess the Mark of Shadow), each house exclusively has one type of dragonmark.
Dragonmarks come in five forms: aberrant, least, lesser, greater, and Siberys. Aberrant dragonmarks are deviations from normal marks and are not recognized by the dragonmarked houses. People who have aberrant dragonmarks are commonly believed to have been warped by Khyber, the Dragon Below.
The source material states that each of the houses are not traced back to a single member bearing a certain mark, rather that certain marks manifested in certain regions on specific races and families. After several generations these families began to understand the significance of each individual mark and came to see certain marks as “true marks”, marks that breed true in a bloodline. Aberrant marks are those that manifest as a consequence of interbreeding of bloodlines.
Ultimately the true bloodlines would wage war on those with aberrant marks in a conflict known as the War of the Mark. The results of this conflict would unite the true bloodlines officially into the current houses represented and lead to a founding of the council known as the Twelve. The Twelve would become responsible for codifying the traditions of the dragonmarked families and set forth the standards by which the current houses are governed.
Eventually a man who famously became known as Galifar I would succeed in uniting the continent together into the nation of Galifar, the house leaders would come together in the city of Korth and make arrangements with the would-be-king that would limit their political power in exchange for assurances of their continued economic strength. These would become known as the Korth Edicts and would last for a thousand years and were designed to prevent members of any of the dragonmarked houses from creating standing armies (excepting house Deneith to provide mercenary services), disallowing members of the houses from marrying into the aristocracy of Galifar, and insuring that no house could hold a grant of land.
By the time the Last War was in full swing, the Korth Edicts would be set aside allowing house enclaves to maintain forces and by the end of the war several houses now have large land holdings of their own. Many of the houses themselves were said to wield as much power as the nations themselves and it is doubtful any of the twelve new nations have enough power to enforce the terms or renegotiate the edict.
House Cannith Mark of Making Race: Humans Dragonmark Abilities: The Mark of Making grants the power to mend or fabricate material goods.
House Deneith Mark of Sentinel Race: Humans Dragonmark Abilities: The Mark of Sentinel grants various powers related to personal protection
House GhallandaMark of Hospitality Race: Halflings Dragonmark Abilities: ability to manipulate arcane energy to generate shelter and sustenance
House JorascoMark of Healing Race: Halflings Dragonmark Abilities: healing; depending on the size of the dragonmark, the individual can be restricted to healing small wounds, or to curing major diseases, even regenerating lost limbs.
House Kundarak Mark of Warding Race: Dwarves Dragonmark Abilities: ability to lay magical alarms and traps.
House LyrandarMark of Storm Race: Half-Elf Dragonmark Abilities: power over wind and weather.
House MedaniMark of Detection Race: Half-Elf Dragonmark Abilities: bearers of this mark can sense hidden threats, such as poison.
House Orien Mark of Passage Race: Human Dragonmark Abilities: dependent upon the size of the mark, bearers may possess great speed, the ability to fly or even teleportation.
House PhiarlanMark of Shadow Race: Eladrin Dragonmark Abilities: powers over illusion, deception and scrying.
House SivisMark of Scribing Race: Gnome Dragonmark Abilities: power over written and spoken word. Through use of dragonshards, can communicate over vast distances with the speed of the wind.
House TharashkMark of Finding Race: Human and Half-Orc Dragonmark Abilities: can track things that are lost or hidden through use of their mark.
House ThuranniMark of Shadow Race: Elf Dragonmark Abilities: power over illusion, deception and scrying.
House Vadalis Mark of Handling Race: Human Dragonmark Abilities: animals bred by those bearing the Mark of Handling are more intelligent, stronger, faster and more powerful than those bred and/or trained by those without the aforementioned mark.