If you would rather not wait to experience the joy of running through the game as Plague Knight (typically, you have to beat the original campaign first), hold the attack button and enter the following button combination at the title screen: UP, UP, RIGHT, DOWN, LEFT, LEFT. Release the attack button and you should unlock the Plague of Shadows mode. There is an additional challenge mode available, as well. To access that, hold the attack button and press UP, UP, LEFT, UP, UP, RIGHT, UP, DOWN, UP and then release the attack button.