I actually already tried the Ancient target thing and it doesn't work. Turns out that changing the Targets Allowed at all makes no change to the spell.
I have been messing with triggers and cannot figure out any way to do it because there is no condition to specify what kind of damage is being done (aka I need ability damage source not unit damage source). Damage is damage. And I can not use the return damage factor of the ability as a method using math to replace damage on the attacking unit because I don't know which order the ability does damage. For example does it return 40% of FULL damage or 40% of damage done AFTER taking into account armor, attack type, etc. (Tests for dmg aren't consistent)