Tieflings are any kind of mixture of a mortal race and a fiend (demon or devil).

As such, the Tiefling template may be combined with another racial template, or bits and pieces of another. Any player wishing to do this should ask for the GM's permission first. Regardless, the GM should be more involved in the building of such a character.

Tieflings are usually the result of rape, as few mortals would willingly share a bed with a fiend. Because of this, they're usually raised by single mothers. Many spend their childhood ostracized by their peers and have Disadvantages accordingly. Their heritage is normally quite obvious, due to vestigial wings, small horns on the head, possibly a tail, etc.

Their reputation is not entirely without merit, as many Tieflings actually are crazy.