Ok... In order to be able to do that you will need a dummy bladestorm (obviously) with no damage. Now, hopefully you know how to make triggers and so I will pass through very quickly. Once an unit begins casting an ability and ability being cast is equal with whatever you want you should make a for. This for will be from 1 to x, x being the amount of seconds the bladestorm will last. Now, let's see the actions for the for.
Pick up every unit within 300 range of (Casting Unit) matching condition (((Matching unit) belongs to an enemy of (Owner of (Casting Unit)) equal to true) and do Set life of (picked unit) to ((Life of Picked Unit)-(100+strength of (Casting Unit)). Big Pick, eh? Now, you wait 1 second and check: If (((Casting Unit) is dead) equal to true) then you Skip remaining actions, else you do nothing. The only way Bladestorm can stop is if the unit is killed. This should work I guess. You can also set the wait to 0.20 seconds and increase the x five times. Hope it helped you...