Lasting Frost is a paragon tier feat available to characters level 11 or above.
Once per turn, when a character with the Lasting Frost feat hits a target with a cold power, the first target hit gains vulnerable cold 5 after the power resolves. The vulnerability lasts until the end of the character's next turn.[PH:203][U:3/2010]
The March 2010 update limited Lasting Frost to applying to only the first target hit by a cold power per turn, and clarified its timing.
Lasting Frost is an essential part of the frostcheese strategy. Creatures made vulnerable to cold by Lasting Frost grant combat advantage to characters with the Wintertouched feat using cold keyword powers.