In a stroke of luck I have obtained both Khepri's Sting from the night fall and Celestial Nighthawk from a legendary engram. Unfortunately it seems Khepri's Sting have a bug with blade dancer. Escape Artist (Invisibility after a melee) is prevented from working when Khepri's Sting is used from invisibility. So basically what I was trying to do was be invisible then use my Khepri's Sting to do massive damage then be invisible again to deal another blow on another enemy or the same. Escape Artist still works when not invisible. Make sure to spreed the word so everyone knows of this issue. In the mean time I would recommend using Fast Twitch because Backstab does very little extra damage.