Take a look at the below in the ultimate combat book:
Knight of the Sepulcher (Archetype)
Not content with the antipaladin's mere corruption of the soul, the knight of the sepulcher sacrifices mortality along with morality.
The knight of the sepulcher archetype is available only to the antipaladin alternate class.
Touch of the Crypt (Ex) : At 5th level, a knight of the sepulcher gains a +2 bonus on saving throws against mind-affecting effects, death effects, and poison. He is harmed by positive energy effects and healed by negative energy effects as though he were undead, although negative energy effects that don't heal undead (such as enervation) affect him normally. The knight of the sepulcher has a 25% chance of ignoring critical hits and the bonus damage from sneak attacks as though he were wearing armor of light fortification . This ability replaces the fiendish boon ability.