Ice covers the ground, and biting wind makes your foes more vulnerable to your cold attacks.

Daily   ✦     Primal, Zone
Standard Action      Area burst 2 within 10 squares

Effect: The burst creates a zone of difficult terrain that lasts until the end of your next turn. While within the zone, any enemy gains vulnerable 5 cold. You can end the zone as a minor action.

Sustain Minor: The zone persists, and you can increase its size by 1 to a maximum of burst 5.

Published in Player's Handbook 2, page(s) 90.