For the Legion version, see Horn of Winter (Legion).
Horn of Winter is a group buff that death Knights can cast, increasing attack power. The buff should be kept up at all times, as the attack power will increase your damage done significantly. As a tank, the extra attack power will increase your damage (and therefore threat).
Patch 6.0.2 (2014-10-14): No longer generates Runic Power, has no cooldown, and lasts 1 hour.
Patch 5.0.4 (2012-08-28): Now increases attack power by 10% instead of Strength and Agility.
Patch 4.2.0 (2011-06-28): Agility (and thus Horn of Winter) no longer increases a Death Knight's chance to dodge.
Patch 3.2.0 (2009-08-04): Learning rank 2 of this ability caused players to unlearn rank 1, such that it no longer appears in their spellbook. It is now possible to relearn that rank, and death knights will no longer "forget" rank 1 when they learn rank 2.
Patch 3.0.8 (2009-01-20): Now has no cost and grants 10 runic power in addition to its stat buff, but has a 20 second cooldown.