Trigger that detects a hero ability being used?

How do I make a trigger that detects when a specific unit is using a specific skill, and then creates a special effect?

Thanks.

Pretending the spell is holy light, this is how it would go.

Events:
Unit- (Bob the spell caster) is issued an order targeting an object
Conditions:
(issued order) is equal to (holybolt)
Event:
Special effect: Create a special effect on the origin of targeted unit whoopdeedooda.mdx

The problem is that it won't wait until he casts the spell, it will just activate when you tell him to. It would take effect even if the target was all the way across the field. I don't think you can do anything about that. Remember to change the order field in the condition to fit your spell. Remove all spaces, and hope that works. And one more thing, it won't always be the right name. As shown here, to reference holy light you actually have to type in holybolt. I hope all that makes sense!

hmm makes sense. What about for a passive skill such as bash or critical strike? possible?

Nope.

That's why they are called "passive".