Lucent Fortification (Grim Alteration)
From D&D Wiki
His body, flaring with swirling light and energy, made attacking what might have been a weakness, a difficult undertaking.
Prerequisite: Lucent Body (Class Feature), Greater Lucent Body
Description: The grim gains light fortification, granting him a 25% chance of normal damage against special attacks (such as a rogue's sneak attack or a critical hit).