What is the Collar for, and What is a Cavvarach?

The story is set in a world very much like our own, with just a few major differences.  One is that slavery is legal there.  Slaves must wear metal collars that lock around their neck, making their enslaved status obvious to everyone.  Any slave attempting to escape faces the dilemma of how and where to illegally get their collar removed (a crime punishable by enslavement for the remover).  

Another difference is the popularity of a martial art called cavvara shil.  It is fought with a cavvarach (rhymes with "have a rack"), a weapon similar to a sword but with a steel hook protruding from partway down its top edge.  Competitors can strike at each other with their feet as well as with the blades.  You win in one of two ways: disarming your opponent (hooking or knocking their cavvarach out of their hands) or pinning their shoulders to the mat for five seconds.