Cain vs invisible person

I caught it flickering in my peripheral vision in the nick of time, and blocked instinctively, feeling rather than seeing the blow, and was rewarded with the unmistakable whine of diamond-hard teeth slicing into steel.
"Frot!" said a voice close to my ear, an edge of aggrieved surprise in it, and I cut at the source of the sound, but of course my invisible assailant had the advantage of being able to see the humming weapon in my hand...
...
the voice taunted me, and I flicked through a guard position purely by instinct, being rewarded again with another jolting impact as I deflected a strike

Duty Calls pg. 144