Which Diablo expansion introduced the Crusader as a playable class?

Reaper of Souls introduced the Crusader as a playable Diablo class.

The expansion added the Crusader alongside a new act, a higher level cap, the Adventure Mode system, and the Mystic artisan. Crusaders are heavily armored holy warriors who use shields, flails, and divine skills against the forces of Hell.

Their abilities include Blessed Hammer, Falling Sword, and Akarat's Champion. The class is thematically related to the Paladin from Diablo II, but it is not the same class and has its own skill system, equipment, and story background.

Reaper of Souls also shifts the narrative toward Malthael, the Angel of Death. Its campaign is set largely around Westmarch and ends in the Pandemonium Fortress, expanding Diablo III beyond the original game's five-act structure.