Make custom dummy spell based on Shadow strike (if you want it to dead DPS and slow) and custom passive spell based on evasion Remove all the fields of the evasion Rename it to whatever you want. If you're attacking, you'll need Looking_for_help's DDS (I think that's the name). When your unit attacks, check if the level of that custom evasion you created is greater than 0, and set your desired chance by (Random Integer number between 1 and 100 (greater than or equal to "chance")). If it returns true, create dummy unit at enemy position, give it that dummy spell and order it to cast shadow strike onto attacked unit. Make sure you remove leaks.
There are many other ways, and this may not be the most efficient, but this first came onto my mind and it works.