Shatter

Shatter is a mage talent that multiplies the mage's spell critical strike against frozen targets by 2/3. A frozen target is a target immobilized by a frost-based effect, such as Frost Nova, Freeze, Freezing Trap or the freeze effect from the Freezing Band. The critical strike chance applies to all of the mage's schools. The talent is popular in most frost builds and elementalist builds, especially in combination with Frostbite which freezes more enemies. Mobs in raid zones, especially raid bosses, are often immune to freezes making this talent less useful in those situations.

Tips
Freeze effects now have a much higher chance to break when taking damage, meaning it can be difficult to get more than one spell to land on each freeze. It is still possible to get two criticals on the same freeze effect before it breaks. First, use a long cast time spell that also has a projectile travel time, such as Frostbolt or Fireball. Immediately after the cast, use an instant spell such as Ice Lance or Fire Blast, or use Presence of Mind with another spell, if you are fast enough and at good range. The check to see if the target is frozen, and thus the critical strike check, is done at the time the cast completes, while the freeze effect is not removed from the target until after the spell hits. Thus, both spells will have the benefit of Shatter due to the time of flight for the first spell.

This talent is considered essential and the main source of damage for frost mages in PvP.