Exploring the latent potential in your blood due to your fiendish descent, you learn how to better adapt to the mystical attacks of your forebears.
Prerequisites: Spontaneous arcane caster, must be descended from a fiend.
Benefit: Against the attacks, spells, and spell-like abilities of fiends (evil outsiders) and half-fiends, you gain a +2 bonus on your saving throws. This bonus does not apply against the assaults of opponents of any other kind.
Source: Heroes of Horror, page 120.