"These massive weapons of destruction are favored by the zealous Crusaders. Some find the raw offensive power and the ability to pulverize the minions of the Burning Hells with a couple blows to be quite compelling, even at the cost of using their shields."— Game Guide(src)
Two-Handed Flails are a heavier, two-handed variant of Flails used only by Crusaders in Diablo III. Contrary to the lore entry, they are often used with a shield (or a crusader shield) through the Heavenly Strength skill. Flails may not be transmogrified into any other weapon type.