Return fires on the start of attacks though, and deals a set amount of damage. Try using a damage detection system, and whenever a unit takes damage and has the buff, cause it to deal X% of damage dealt to the attacking unit -- this will work for spells as well, if you don't want that, use an attack detection buff (envenomed spears) and add it to every unit, this will however cause buff placers to not stack with it, you can solve it by simply place buffs in the same trigger (if <Hero> has <Item or Ability> then create dummy and make it cast the according ability at the target).
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory