Uff... well then..
You need to know what is a trigger and a dummy.
You need to trigger the spell using trigger editor. make a new trigger and name it.
And it gives you events/conditions/actions
First you set the event to "unit startrs the effect of an ability"
Condition: Ability being cast equal to <your base ability here>
Now you need to create a dummy, go to object editor and make new unit(take footman)
now remove the model and write .mdx in the model path, remove the shadow, turn off collision disable attack, give locust and invulnerable abilities to it.
Actions: here you set some locations where you create a dummy.
as you create a dummy give abilities to it (one with 10 damage second with 8 damage...)
Then you order it to cast those abilities on the targeted unit.
Sorry its hard do explain everything, you should read some tutorials about mapping, triggering.
You can find some really good tutorials here in Tutorial sections =)