Hoarfrost is a powerful attack used by the Lord of Destruction, Baal, during the final battle against him in the Worldstone Chamber. This attack inflicts cold damage that slows the player when hit. It also has a knockback effect that is continuous as long as the player remains in the skill's attack path, making it very difficult to maneuver, much less fight.
While Baal uses it quite frequently, it can be easily avoided by the Barbarian's Leap skill and the Sorceress Teleport skill. When Baal uses this skill consecutively, however, it is more difficult to avoid. The best defense is, if possible, to back yourself up against a wall or pillar.