What is "tiefling"

The tiefling, in the Dungeons & Dragons (D&D) fantasy roleplaying game, is a humanoid race. Originally introduced in the Planescape campaign setting in the second edition of Advanced Dungeons & Dragons, they became one of the primary races available for player characters in the fourth edition of the game.

Tieflings were originally introduced as humans with demonic ancestry, whose lineage can be traced back to some degree or another to that of a fiend or demon within the Dungeons & Dragons universe. A tiefling should not be confused with a half-fiend, as half-fiends are generally half mortal and half fiend or demon, whereas the demonic lineage of a tiefling's ancestry often lies further up the family tree.

In 4th Edition Dungeons & Dragons, Tieflings are a race whose human ancestors made a bargain with devils to increase their power. Their origin is similar in 5th Edition.