The yuan-ti are a species of evil snakemen. The species comprises a number of castes. In some campaign settings, the yuan-ti are descended from evil human cultists who mixed their bloodlines with those of serpents. The original yuan-ti castes were the abominations, the halfbreeds, and the purebloods, which first appeared in the module Dwellers of the Forbidden City (1981), for the first edition of Advanced Dungeons & Dragons. Just as the beholders, displacer beasts and mind flayers, the yuan-ti are considered «product identity» by Wizards of the Coast and are not released under its Open Gaming License for this reason.
Purebloods are described as human-like but with minor reptilian features, such as snake-like eyes, a forked tongue, and patches of scales on their skin. They have weights and heights in the same range as humans.
Among the yuan-ti halfbloods, there is great variety. Not only they come in several general body types, but they also tend to show variations in scale patterns and colors, much like natural snakes. Four in ten halfbloods have the body of an agile, sharp-featured human but with a serpent head and scales. Some varieties have hoods like cobras. The remaining 60% of halfbloods fall into three remaining variations. Some had the lower body of a snake instead of legs, others had a snake-like tail, and still others had snake-like arms in place of normal arms. Halfbloods also tend to have body masses and sizes in the same range as humans.
Abominations, though, have almost completely serpentine form and vaguely humanoid arms able to use weapons and tools. They are the form of yuan-ti that most closely resembled the originally intended appearance of the yuan-ti race as envisioned by the serpent gods.
Yuan-ti society as described in «The Ecology of the Yuan-Ti», Dragon #151 (1989) is divided into the pureblood, halfbreed and abomination castes. The histachii were described as the progenitors of the yuan-ti. It was necessary for the yuan-ti to acquire human captives to turn into histachii, as breeding between any of three yuan-ti castes resulted only in abominations, while histachii could produce purebloods and halfbreeds.
In 5th edition, there is no god who patrons the yuan-ti, but the Monster Manual lists Dendar, the Night Serpent, Merrshaulk, Master of the Pit, and Sseth, the Sibilant Death, as their gods, and reinvents them as a transhuman species who originally viewed snakes as the epitome of logic and stoic behavior, building cults to the serpent gods who gave them the knowledge to ritually breed with snakes, creating their race. They seek to rebuild their ancient empires through manipulation, such as awe, pleasure and terror, promising wealth and power. They build cults and seek to achieve great power in order to devour and supplant their gods.
While the yuan-ti do not feature as prominently in Eberron as in some other settings, they still have some significance and influence, especially in Xen’drik. Yuan-ti in Eberron worship the Devourer. No yuan-ti are known to live in Khorvaire, with the exception of the shulassakar. If any do, they probably live in warm marshy or forested areas, such as Q’barra and the Shadow Marches.
The yuan-ti have many forest and underground bases in Xen’drik, since it is their main power base on Eberron. The yuan-ti of Xen’drik create wretchlings, ssvaklors and magebred vipers to guard their strongholds on that continent.
The shulassakar, or feathered serpents, are a race of lawful good-aligned yuan-ti native to the ruins of Krezent in the Talenta Plains. The shulassakar revere the couatl and the Silver Flame. They resemble yuan-ti, but are covered with feathers (and, in some cases, wings), emphasising their connection to the couatl. Pureblood shulassakar are called bloodsworn, halfbloods are called flametouched and abominations are called transcendents. It is unknown if other kinds of shulassakar exist. You can learn more about the relation between shulassakar and yuan-ti in Keith Baker’s Dragonshards article about them.
By far the most powerful and loathsome of yuan-ti, anathemas are worshiped as divine incarnations of Merrshaulk. They are truly bizarre in appearance, with a body like a 25-foot-long serpent; a pair of humanoid arms with clawed, three-fingered hands; and six serpentine heads rising from its shoulders. An anathema’s power dwarfs even that of an abomination, but since they represent such unfettered chaos, they are usually found away from yuan-ti cities, establishing cults that worship Merrshaulk through them. They make a common practice of grafting yuan-ti body parts onto humans and other races for use as personal servants. Anathemas are extremely rare, however, and only a handful are likely to exist on any world.