Not rare. It is guaranteed to drop off skeleton king in campaign mode (1st kill). Should be lvl 16 or higher to make sure it has the gem slot (not sure if this has been changed, but that's how it used to be).
I get it all the time below lvl 10 with socket, I don't think I've ever seen one drop without a socket. At lvl 8ish, get a bounty to kill Skellie King, it usually drops....and then it drops off of everything else - only good for cube after more leveling tho.