Once certain criteria are met, players will be able to create a new death knight character, which will start at a high level.
The death knight combines martial prowess with dark, necromantic energies. Players might be familiar with the death knight from previously released Warcraft games -- most recently, in the campaigns for Warcraft III: Reign of Chaos and the Frozen Throne expansion, the former paladin Arthas became a death knight and wreaked havoc across Azeroth and Northrend before fusing with the spirit of Ner'zhul to become the Lich King. The death knight is the first hero class in World of Warcraft; once certain criteria are met, players will unlock the ability to create a new death knight character, which will begin play at a high experience level. Blizzard is currently exploring various options for how the death knight will play and the types of abilities at its disposal, and they'll be revealing more information about this new class as we get further along in development. The way death knights were portrayed in past Warcraft games is certainly a source of inspiration for Blizzard, but they are still determining how best to capture that feel within the mechanics of World of Warcraft, and haven't made any final determinations on specific abilities. In general terms, the death knight can be considered a type of class that combines damage dealing and tanking, but naturally it will have an array of unique abilities as well.
Races
Currently, Death Knight is planned to be available to every race.
Weapons and Armor
Death Knights will be able to use two-handed weapons and dual-wield.
Controversy
Death Knight response by community
Many players have expressed concern over the introduction of Death Knights for several reasons. Blizzard has said that Death Knights will be the only hero class in Wrath of the Lich King, so they could quickly become overabundant, especially due to their popularity, and relatively high starting level, not requiring as much time or commitment (exact level not decided yet). Their projected role may overshadow existing classes such as warriors as well, which could easily lead to balance problems.