Thorns Help Please

I want to make a spell called Spear Wall. It will be much like defend. When it is activated, the unit will become slower in walking. But instead of receiving increased defense, I'd like it to activate a thorns aura that only affects that specific unit. Once I disable it, the aura goes away. I saw this in a Castle Builder map and was trying to duplicate it. Thanks

Hi

Maybe you could use the defense ability modified to give no defense, to activate a trigger that would give the thorns aura to the caster unit, when deactivated the thorns would be removed.

That's a wonderful idea. However, I'm not so hot with triggers, so do you think you could help me out?

Ok
I have worked on a map that I will put here for you to see how I did it.

Basically I made Warcraft give the thorns aura to a unit that used rhe order defend, because this order is issued when a uni uses the defend ability, it is easy to understand how it works, to remove the thorns, there is a trigger that is activated when there is the order undefend.
This way you don't need to work with "begins/finishes/channels/finishes casting an ability" which would be more complicated

Hope it helps