You will have to trigger this - it will be simple trigger, but to make it bug-proof, you will need to use Damage Detection System (DDS).
The reason why:
You need to correctly catch when some unit deals damage.
Units dynamically come in and out of range of your aura, so you cannot use the even "Unit - unit takes damage", as that requires a specific unit (and if you do it this way, the trigger will become very inefficient).
If you do it via event "Unit - unit is attacked", then players may abuse it by stopping the attack before it hits (you could replenish mana to full by abusing this).
Damage detection system is system created by people for other people - there are some systems here on hive (for example this:
http://www.hiveworkshop.com/forums/...earch=damage%20detection%20system&d=list&r=20) - you will have to import this system into your own map (these maps contain instructions on how to import and use it).
Once imported, create your own custom aura spell (based off some existing one) and null its fields, so units get affected, but don't get any beneficial effects from it. Create your own custom buff and give it to the aura.
Then comes the trigger: When unit takes damage, you make a simple trigger which checks if the damaging unit has the buff from your aura. If yes, you replenish part of its mana, else you do nothing.
It's unfortunate that otherwise such simple spell requires a whole system to work correctly but that's how WCIII is :/
