Shatter is one of the Mystical abilities. You need 16 Mystical use points to be able to cast it. If you have 15 use points plus your bonus 1 pt for being a Mystical user, you can still cast Sh even though it won't show up on your M menu. This is true for every M ability that costs multiple points.
Shatter is the most powerful offensive attack in the entire game. It does 4.5x the damage of a regular attack. Shatter is used nearly exclusively in player fights, though it can be used on forest monsters and the dragon.
|Damage = [(YourStrength/2) + rand(0, (YourStrength/2) - 1)] x4.5 - EnemyDefense. (Round down fractional damage)|
Strategic use Edit
You can get more exp faster if you have Shatter in your arsenal. Shatter is so powerful, users can usually take out a player two levels above them with Sh + T, provided the target doesn't have excessive equipment. Sometimes Light Shield is also needed. Competitive players take on these challenges with Shatter because usually higher targets have much more exp to share when killed.
Shatter is not considered an efficient use of M skills on the dragon, in most cases. This is due to the dragon's massive 15,000 HP. Shatter will only take a fraction of that, and will use up points that could better be used as Mind Heal. And no one wants to waste points that could have gone for Mind Heal. The exception to this would be when you have between 28-31 M points when you face the dragon. You can throw on LS and then Shatter the dragon, then begin your T uses and any extra pinches, and hope for the best.