Collision Spell

How would I..

Create a spell that makes a unit able to "phase" through other units for a set amount of time. I know that I would have to create a dummy spell that sets off a trigger, but how would I make the trigger work? I know that modifing its collision size can do weird things, but I just want the unit to pass through other units, not trees and stuff.

Ive already made a spell like this, simply do this... make a spell based off of summoning something, then make a trigger that detects the casting of the spell, in my case this works since i had only one unit who could use the spell..
E:
Unit enter playable map area
C:
Unit type triggering unit is (type you made spell for)
A:
-Start timer NonC (my timer variable for the spell) as one shot that expires in X time
-Turn collision for Shade off... (replace shade with the unit you have who can cast it..)
-Turn Vertex shading to 100%, 100%, 100% with 80% transparency (just an effect not needed)

then next trigger
E:
-NonC expires (meaning spell is done)
A:
-Turn collision for shade on (or whatever unit)
-Set vertex coloring to 100%, 100%, 100%, 0% transparency.

Now when they use the spell they will turn ghost like and be able to pass through ANYTHING they want, until the time runs out. Be careful this means they may leave the bounds you think they are limited too. If you mean to have more than one unit use this your going to have to work a lot harder, and my version wont work.

Hmm. THanks. I suppose I can just do a unit order=spell for the unit, and make it all shade like. I wish I could just make it pass through just units *shrug*. I'm assuming it passes through doodads too, but will cliffs and map boundaries stop it?

Unfortunately, no. Cliffs definitely dont and boundries Im unsure of but I think they dont stop em either.. this is why I had to remove this spell from my map as well, the people with the shade just kept leaving the arena and attacking from outside... So Im sorry if that makes this hard for your map, but I dont know of anytihng that can be done about it.