Winter's Mark

Whenever the wielder of this +5 frost two-bladed sword hits a new Medium or smaller enemy for the first time, the enemy suffers additional 2d6 cold damage. Whenever the wielder hits a new Large or bigger enemy for the first time, the enemy must pass a Fortitude saving throw (DC 30) or be surrounded with a frost aura, making all other enemies in a 15-foot radius become vulnerable to cold damage for 3 rounds.