I would like to once again remind, that shadow image ability of nightblades is bugged, and has been since 3.2.8.
This ability is one of the most vital abilities for nightblades, both stamina and magicka (but especially magicka) in terms of survival in pvp, it needs immediate attention and really can't wait till next update to get fixed, as nightblades (again, especially magicka) don't have much in their kit for survival in pvp, shadow image is not really replaceable.
@ZOS_GinaBruno ,
@ZOS_JessicaFolsom ,
@ZOS_Wrobel
Here is a video showing the bug.
https://www.youtube.com/watch?v=3dpMQfd3BTg