Tieflings are humans with an ancestor of evil-aligned outsider blood, usually a devil or demon, and carry the taint of this dark heritage within their souls. The legacy of a distant ancestor may show up in the bloodline many generations past, thus tieflings could be born to a couple of otherwise normal humans. They innately understand that they are different, whether or not they are aware of their ancestry, and often have strange urges and desires because of this taint. Those that are physically noticeable are feared and outcast throughout the realms, if not killed at birth. Most succumb to their darker lineage and tieflings have a natural disposition towards the dark, as influenced by their ancestry. A rare few follow a path of twilight.

Tieflings should not be confused with half-demons or half-devils, who are a different species, rather than a subspecies of human. They have the appearance of a human except for one or two physical attributes that carry over from their lineage. Some examples are:

• small horns
• forked tongue
• glowing-red or cat-like eyes
• hooves or goat-like legs
• more or less than five fingers
• non-prehensile tail
• hot, furry, leathery, scaly, red, or bruised blue skin
• scent of brimstone
• casts no shadow or throws no reflection