While it doesn't speak to the weapon's historical existence, fantasy author an Society for Creative Anachronism founder Paul Edwin Zimmer wrote at length about

While it doesn't speak to the weapon's historical existence, fantasy author an Society for Creative Anachronism founder Paul Edwin Zimmer wrote at length about a 'war flail' consisting of a wooden handle attached by a very short chain to a long, triangular metal bar that was used as the striking portion of the weapon. I can easily picture how a 3' metal bar, spun as fast as a strong and practiced man could move it would be intimidating and deadly, shattering swords and shields, and chopping through any possible armor with ease.