The Death Knight blows the Horn of Winter, which generates 10 runic power and increases total Strength and Agility of all party or raid members within 30 yards by X. Lasts 2 min.
The Death Knight blows the Horn of Winter, which generates 10 runic power and increases total Strength and Agility of all party or raid members within 30 yards by X. Lasts 2 min.
Horn of Winter is a group buff that Death Knights can cast, increasing strength and agility. The ability also generates a small amount of runic power when used. The buff should be kept up at all times, as the attack power gained from the strength and the critical strike chance gained from the agility will increase your damage done significantly. As a tank, the extra strength will increase your damage (and therefore threat), as well as your chance to parry through Forceful Deflection, while the agility will boost your armor and chance to dodge.
Horn of Winter does not stack with shamans' Strength of Earth Totem, nor does it stack with scrolls that increase strength or agility. It does however stack with Mark of the Wild and all effects that increase strength and/or agility by a percentage, such as Blessing of Kings. The extra attack power gained from the strength is also affected by effects such as Abomination's Might.
The spell grants runic power to you even if you have a more powerful effect already on you preventing the buff from being applied (such as a shaman with talents to improve their Strength of Earth Totem).
Horn of Winter is one of the death knight's three abilities that generate runic power at no rune cost, the others being Blood Tap and Empower Rune Weapon.
Horn of Winter should be used before every pull for the free runic power. It should also be used any time you have a spare global cooldown to generate some extra runic power and to make sure the buff does not fall off. This applies even if a shaman in the group is using Strength of Earth Totem, as the extra runic power is always useful.
If you are grouped with a shaman, you can ask the shaman to use Stoneskin Totem instead of Strength of Earth Totem. In addition to granting extra armor to the tank, it will also increase your attack power if you have the Bladed Armor talent. Note however that Stoneskin Totem does not stack with a paladins' Devotion Aura.
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.
Edit
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. 
