Which expansion introduced the Death Knight hero class?

Wrath of the Lich King introduced the Death Knight hero class. Blizzard added the class with the expansion's release in 2008, giving players a new way to enter Azeroth as powerful undead warriors connected to the Scourge.

Death Knights were unusual because they began at a higher level than ordinary characters and started with a dedicated quest storyline in Acherus: The Ebon Hold. The introductory campaign showed their service under Arthas before they broke away and joined either the Alliance or the Horde. Their abilities combined martial combat with runic magic and diseases.

The class could fulfill tank or damage-dealing roles and originally had three talent specializations: Blood, Frost, and Unholy. Later class and specialization changes altered how those roles were arranged. The Burning Crusade added no new class, while Cataclysm changed existing zones and added the worgen and goblin races. Demon hunters became World of Warcraft's second hero class in Legion, following the Death Knight's debut in Wrath of the Lich King.