As a reaction, which you use when you are hit by an attack or taking necrotic damage, you absorb some of the incoming energy as shadowy tendrils erupt from your body. Until the start of your next turn you have resistance to necrotic damage and the shadows turn dim light within 2 meters of you into darkness, and bright light in the same area to dim light. If the creature who damaged you is within 10 meters, it must make a Reflex saving throw.
On a failure, it takes 2d10 necrotic damage and is pushed or pulled 2 meters away from you or toward you if it is Medium or smaller.
On a critical failure, it takes twice as much damage and is pushed or pulled 4 meters away from you or toward you if it is Medium or smaller.
On a success, it takes half as much damage.
On a critical success, it takes no damage.
You can increase the damage by 2d10 and affect a creature of a larger size for each additional mana expended.