Tiefling is a blanket term for individuals that are touched by fiendish influences, and they have never had a large population in Eberron. Most tieflings are born to parents of other races, and they are exceedingly rare. They are also incredibly diverse, the spontaneous nature of their birth and the variety of creatures that can produce them means that tieflings are as varied in appearance as they are in origin. One thing ties them all together, they were all touched by malevolence before they were born, and this influence gives them innate magical powers. The supernatural evil that formed them does not influence their behaviour, but it does dictate the nature of the powers they will manifest as they mature.
Most people in Khorvaire will never see or meet a tiefling, they are closer to creatures of legend than something that anybody expects to actually encounter. The rise of Droaam has familiarised most people with the idea that monsters are no more inherently evil than anybody else, and people in Khorvaire are fairly used to seeing monstrous individuals in their day to day lives, so tiefling's are unlikely to be treated any differently than one would treat any other denizen of Droaam, whether they are recognised as tieflings or not.
Whilst every tiefling is formed under unique circumstances, they can broadly be categoried. The most common origin for tieflings in Khorvaire is planar influence, and these tieflings are known as Planetouched. Kaluunite Tiefling originated from the Sarlonan nation of Ohr Kaluun, and fled to Khorvaire during the Sundering. Finally, whilst all tieflings are touched by malevolance, none are more so than the Sakah, who are shaped by the essence of one of the Overlords.
Thrane has a sizeable population of tieflings, mostly concentrated within the town of Rellekor.