The Tiefling, or Fiendkin are a race of Elf-kind with devil or demon blood in their family tree. Sometime around 9000 QA, the Drow, jealous of the Immortality of the surface-dwelling Elves, looked for ways to expand upon their own lifespan. This lead to some drow contacting Demons, and, through science and magic, integrated Devil blood into their own. This new race was shunned by the Drow, becoming their own people shortly after the initial experiments. Despite the great risk, Tiefling lives are no longer than those of most men and Drow.
While not inherently evil, Fiendkin have an inherited tendency toward evil and, therefore, are mistrusted at best and outright persecuted by some communities when they are discovered. A Fiendkin will almost always be put to death or abandoned before it is raised by a community, although exceedingly rare exceptions do exist. Some may confuse them as proper devils or demons, either worshiping or zealously murdering them, depending.
Largely unknown within the Reach of the Imperium, legends of great Fiendkin-cities (Ubi'diabolus'est to the People) where Devils cohabitate with man, exist within some of the oldest myths of Elven kind, possibly dating to a time before the Elves arrived on the surface and split from the Drow. Any and all attempts to find these legendary Ubi'diabolus'ests have failed miserably, to the point that to embark on an impossible quest has come to be called "seeking the devil's city."