The sword itself has a +1 attack bonus, and adds 1d4 cold damage when it hits. When you roll a 6 on a d6 or a 4 on a d4 on a damage roll with this weapon, you can inmediatelly roll again a dice of the same kind and add it to the total.
Furthermore, when a critical hit lands on a creature, this must roll a DC 15 Constitution save. On a failure, the creature suffers 3d6 cold damage and its covered in ice crystals, paralizing it. The creature can repeat the save at the end of it’s turns. If it succeeds damage is halved and it’s not paralized.