Midwinter Chaos is a rare spell that freezes all weak enemies and destroys the weak ones that were already frozen. The strength limit of Midwinter Chaos can be increased by leveling it up. Midwinter Chaos fits well into freeze and control decks.
Midwinter Chaos' effects of mass freezing or destruction is its key value.
Combining Midwinter Chaos with Wisp Cloud is a great way to get rid of swarms. A cheaper method of area damage is to use Frosthexers and have Midwinter Chaos do the destruction instead.