Temporarily/Permanently adding variable damage to attacks

[Big Dub]

Ok, there are two things I want in my map. I want a spell that as adds a percentage of your missing life to your attack damage. I also want an item that adds more damage to your attacks for each unit you kill while you have the item.
I know how I would work a lot of it. The problem is actually adding the damage to the units attacks, I would like it if the players could see the added damage in green next to their original damage.
Any suggestions?

 

[MortAr]

Item - Item Bonus Damage w/ or w/o the hidden spellbook trick.

 

[arielavi]

what about this hidden spellbook trick? im trying to know something about that, can you post some link to this?

 

[Big Dub]

I was considering that but the idea of having a spell with 100+ levels or so would be rather irritating. Can you give a unit the item bonus damage multiple times? I know you can duplicate abilities on items, but does it work on units? Then how would you remove certain amounts?