% back to attacking unit

I made a spell where a target unit takes back x% of damage dealt when attacking back to himself

The thing is, I need to detect how much damage he dealt, I remembered some way to do it with JASS, but since I'm only begining to JASS, I don't know how to do it yet.

Can anyone help me?

jass? Just give the other guy thorns aura, and have it only be on him.