Winter ice

Winter Ice is the power of Winter. It is the opposite of Summer Fire.

Description
Although not very different from regular ice, it dissipates faster, and is more easily summoned by creatures of Winter, as Dresden begins to use Ice more frequently. This substance provides creatures of winter with absolute sure-footing when standing upon it, though as the Winter Knight Harry benefits from this effect on all types of ice.

Harry Dresden can call upon this power when he becomes the Winter Knight. He describes it as "Cold, clear power," which can augment his spells and remove his weariness when called upon, though it is more effective when creating ice, as Dresden demonstrates when he uses it to transform the floor of Lake Michigan into an ice sheet which lifted him and his friends out of the water. All Winter Faeries have it, and they are susceptible to it, making even the Winter Queen no stronger than the average Faerie when facing the Winter Knight.

Harry Dresden is the only person known to have wielded both Winter Ice and Summer Fire.

Summer Knight
In Summer Knight, Lloyd Slate, the Winter Knight, uses Winter Ice to kill Ronald Reuel, the Summer Knight. It is presumably also used by the armies of the Winter Court in the Battle Over Chicago.

Proven Guilty
In Proven Guilty, the entirety of Arctis Tor is made from Winter Ice.

Changes
In Changes, Harry Dresden uses Winter Ice to kill Duchess Arianna Ortega at The Showdown at the Temple.

Cold Days
In Cold Days, Harry Dresden uses it throughout the novel, being become heavily dependent upon it in his capacity as the Winter Knight.

Skin Game
In Skin Game, Harry Dresden uses Winter Ice to defeat the Ghouls attacking him as he tried to escape with Harvey.