The Hand of Vengeance
The Hand of Vengeance is one of the 4 Horde Expedition factions added in Wrath of the Lich King. The Hand of Vengeance is a Undead faction. The reside in Dragonblight and Howling Fjord mainly but have been seen out of there. Their main hub is Vengeance Landing in Howling Fjord, to get there you can take the Zepplin from Undercity.
There are two ways for players to raise the Hand of Vengeance reputation. One of these ways is by completing all the quests in Howling Fjord and Dragonblight from them. The other way is by not wearing a faction specific tabard while running a Northrend Heroic (added in patch 3.3).
I'd like to also note, to prevent confusion. It will show your Horde Expedition faction and your Sunreavers Faction increasing but not The Hand of Vengeance. If your Horde Expedition faction and Sunreavers faction is exalted then it will display your Taunka faction as increasing. However if you're not exalted with Sunreavers and the Horde Expedition, you will still gain Hand of Vengeance faction.
It's confusing I know. But you'll understand once you see it happening.