Akkhan

"It is said that the cleric gave these crusaders a seemingly impossible task: to scour the far-flung lands to the east and find a means, however it might manifest, to purify the Zakarum faith."

Akkhan was once a mid-level Zakarum priest. At the same time as when Rakkis and his forces departed for the West, Akkhan sensed Mephisto's taint on the Zakarum Church. He did not know that the Lord of Hatred was the cause, only that something was corrupting his beloved faith, and believed that everyone in the hierarchy was corrupt in some way.

Akkhan's standing in the church was low enough to avoid being part of the rituals that would have exposed him to Mephisto's corruption, but high enough to have enough influence to found a new order, for without any real knowledge of the source of the corruption, he couldn't take direct action. In response, he independently (and secretly) founded the Crusader Order, after searching far and wide for warriors of unparalleled strength, for believers who burned with inner Light. He made sure to avoid recruiting anyone with Paladin training, fearful of any existing defilement or corruption carrying over into his crusade.

After rigorous preparation, Akkhan sent this new order into the east to find a way to purify the faith, after preparing a set of writings upon how the Crusaders should be armored and equipped for their crusade. He meanwhile stayed behind to try to fight the corruption directly, a task which eventually cost him his life.

In-game
The Armor of Akkhan set, Akkhan's Manacles bracers, Akkhan's Leniency and Akkhan's Addendum flails can be found in Diablo III. He is also referenced in flavor text of Crusader skills.