"The Heavy Flail has the ability to deal massive crushing power while still being just as unpredictable as the normal flail. It sacrafices speed for its power, but makes up for it in intimidation"
The Heavy Flail is a primary weapon for the Knight. It is a heavier version of the flail.
Instead of having a stab, it uses a shield bash akin to the one used by Javelin-wielding Archers.
It was implemented in the Janurary 2013 patch.
Trivia
The Warhammer does a bit more damage, is twice as long, and is only slightly slower. However, this weapon's biggest benefit is it's unpredictability due to its animation.