The Crusader is a new class for Diablo III that was introduced in the expansion Reaper of Souls. Diablo III is an action role-playing video game developed and published by Blizzard Entertainment.
From the Diablo 3 Game Guide for Crusaders:
Crusaders are unbending champions of faith and law. These living fortresses use impenetrable plate and towering shields to wade through scores of foes, leaving demonic bodies smoldering in their wake.
The thick of battle is unforgiving, but crusaders rush in without hesitation, relying on holy magic and heavy armor to guarantee victory. Well-trained crusaders are adept at deflecting attacks entirely, often shrugging off massive blows that would fell lesser combatants. If pressed, they can sacrifice speed and mobility for pure staying power.
Bone-crunching flails and wicked shields are perfect for melee, but crusaders are hardly limited to extinguishing evil at close range. When one of these divinely empowered warriors joins battle, blazing fire and blinding light follow, smiting entire packs of enemies who dare to resist judgment.