Fiendish Appearance (Ex): The eidolon appears as a fiendish creature and manifests some of the abilities of a fiend. Spells and effects that target creatures with the evil subtype or have specific effects against such creatures affect the eidolon as if it had that subtype. The eidolon gains a +2 bonus on saving throws against acid, disease, fire, and poison spells and effects. It also gains an amount of spell resistance equal to 5 + its HD against spells with the good descriptor. The summoner must be evil-aligned to select this evolution.

At 7th level, by spending 2 additional evolution points, the summoner increases the bonus on saving throws to +4 and extends the spell resistance to affect any spells and spell-like abilities cast by good creatures.

At 12th level, by spending 2 additional evolution points, the eidolon gains immunity to acid, disease, fire, and poison. Its spell resistance increases to an amount of equal to 11 + its HD. (The summoner must pay for the 7th-level upgrade before paying for this 12th-level upgrade.)