The Protection of Shadows is a spell which creates a softening cushion of shadows around anything that is going to strike a critical hit against any party member within 30 ft of the spell caster. This is cast as a reaction and can only be used once per round at 3rd Level for spell casters.

For a Monk this is learned on The Path of the Souls Shadow, and enables the Monk to use 1 Ki point to pull the shadows and wrap them around the object that is striking the critical hit
The attack still hits and normal damage for the attack is dealt to the intended target, however the extra critical damage is negated by the shadowy cushion.