This article's content is accurate to the most recent public game version, 0.4.8d.
"Attempts to destroy an enemy from the inside with ice. Deals high ice damage, 50% chance of Instant Death."
Cost: 12 MP
Source: Equipping the Ice Core.
Frost Shatter is a spell that deals strong 
Ice damage with a 50% chance of inflicting 
Instant Death to a single enemy target.
This spell is basically an ice version of Natalie's Grasp of the Pit. Like Grasp of the Pit, it too deals a nice amount of single target damage and its Instant Death effect stands a good chance to first-strike kill a particularly annoying minion or mini-boss. It also shares the main issue that all the really troublesome enemies like bosses are totally immune to it.
The following enemies can cast Frost Shatter: