Your pact grants you additional benefits when fighting in battle.
Prerequisites
Fiendish Pact, base attack bonus +5.
Benefit
All of your weapon attacks (including natural weapons) are considered evil-aligned for the purposes of overcoming damage reduction.
Additionally, when you apply the bonus from Fiendish Pact to an attack roll, you deal an additional 1d6 profane damage. This profane damage is increased by 1 for each Fiendish Feat you possess.