As a reaction, which you use when you are hit by an attack or missed by 4 or less, spectral frost rimes the creature who hit you. If the creature who damaged you is within 10 meters, it must make a Fortitude saving throw.
On a failure, it takes 2d8 cold damage and is slowed 1 until the end of its next turn.
On a critical failure, it takes twice as much damage and is slowed 2 until the end of its next turn.
On a success, it takes half as much damage.
On a critical success, it takes no damage.
You can increase the damage by 3d8 for each additional mana expended.