Twisted Innards

For the duration of this spell, your vital organs writhe, shift, and move about, making it difficult to strike you in a vulnerable area. While this spell is in effect, critical hits and sneak attacks against you have a 25% chance of failing to inflict any additional damage--though you still take the normal damage from the attack. At 7th level, the chance to ignore additional damage increases to 50%, while at 13th level the chance increases to 75%.