This method also got me my first and only Vex Mythoclast few months back. This may seem strange, but this can only happen when you are desperate. Before you kill the knight, leave a sliver amount of health just enough to drop it with a burst. Now this is the IMPORTANT step, you have imagine getting the husk of the pit dropping to the right side you your screen and simultaneously kill the knight. BOOM, it will appear as it fills your blank imagined husk of the pit, thus becoming a reality. This can be considered the blending of reality and illusion into one entity. When mine dropped, I was doing it on lvl 24 difficulty.