"The Flail is the least predictable weapon in the game. It has mid to short range, and is the fastest knight weapon. It does reasonable damage, but it is difficult to guard against."
Instead of having a stab, the Flail uses a shield bash akin to the one used by Javelin -wielding Archers.
It was implmented in the Janurary 2013 patch.
The Flail and Heavy flail allow the Knight to use a Heater Shield , a shield not normally accessable for the class. And is one of the few unique animations in the game to not have a character bash the weapon with any shield.