A dummy unit is just a unit with no model (or a blank one), that uses the ability Locust to make in un-selectable.
So, just make a unit in the object editor, remove all of the relevant fields (set them to 0), like food cost, and whatnot.
Then you can create it at locations using
Triggers, and order it to cast spells. For more thorough information, take a look at some tutorials.
This, might help you with some of the triggering aspect.
Also, creating two almost identical threads within a day of each other was a bad idea. (
Here), so keep that in mind for the future.