The Death Knight was introduced in World of Warcraft: Wrath of the Lich King, specifically in Patch 3.0.2 as a hero class; initially, this class was only accessible to players who had leveled a character already to Level 55 on that realm. In Patch 6.1.0, this limitation was removed. Players could also initially only have one Death Knight per realm; this limitation was removed in 5.3.0.
When players select this class for play initially, the newly-formed character already starts at Level 55, and by the end of the introduction quests, will be at least Level 58, with gear suitable to immediately start questing in Outland.
Death Knights have three class specializations to choose from: Blood, Frost and Unholy. When the class originally launched, all three could serve as viable tank specs. However, with the launch of the pre-expansion 4.0.1 patch for World of Warcraft: Cataclysm, the specs were re-tuned; Blood was now the dedicated tanking spec, with Frost and Unholy positioned as dedicated damage-dealing classes.