Spell Trigger Help Please

Ok, I' am working on this spell for one of my maps based on shockwave but it shoots out the Banshee projectile Art for Possesion. I made it so that only the Target unit can get hit by this new spell. Im trying now to make a trigger so that when the Unit gets hit with that spell it turns into a banshee(Ghost Spirit).
What i cant figure out is how to set-up the trigger for this to work. I would appriciate all the help I can get thank you.

Hmmm, Ill take a look at it. Ill post something if I can find a way to set it up.

I'm pretty new to trigger editing, so bear with me (I'm fairly familiar with this style of programming, having learned Turbo Pascal in H.S.). Here's what I would do:
1) Make a point variable and set it equal to the position of the caster <We'll call it PlayPos>
2) Make a real variable <Angle> and set it equal to the angle between the position of the caster and the position of the target (I think it's a targeted spell, correct?)
3) Create two variables, one a real <CountReal>, one an integer <CountInt>. Set the real to 1.00
4) Create an array of point variables <PointHolder> (will hold every point on a straight line from the caster to the target)
5) Make a loop that runs 700x (I believe it would be a For loop) that will do:
a)Set PointHolder[CountInt] = (PlayPos offset by CountReal towards Angle degrees)
b)Set CountInt = (CountInt + 1)
c) Set CountReal = (CountReal + 1.00)
d) Run a trigger that converts all enemy units within 67.50 <diameter will be range of Shockwave> of PointHolder[CountInt]

The only problem with this technique is that it will convert units around the beginning and the end of the spell that weren't actually in range of shockwave (because it uses a circle around each point as its AOE). Sorry, that's the best I could do (I've only made one trigger myself, so far). Maybe someone else could finish this trigger fix.