How to let someone take damage

I know this sound extremely stupid but...
How can I give a unit damage? I know how to set his life both in numbers or %. But I don't know how to give him damage.

I wanted to give him damage when he casted a spell. (Instead of spells cost mana, they cost health)

Sorry for such a stupid question...:(
Dark Dragon

E- Spell being cast yadayadayada
C- Spell equals to yadayadayada
A- Set Life of casting unit to (Life of casting Unit - 100)

Right, now that is my problem. I can set his life to whatever I want, but you cant see how much health he has. I cant say that he takes 50 damage (or whatever)...I can only set his life.
...I'm hard to understand, right?

Setting his life to 50 less than he has right now will look like he´s taking damage. I don´t understand what´s your problem sorry.

Hmm..Then i could have just been mistaking because I think/thought that if you use 'Set Life' you will set his life to a certain amount.
Probably means I didn't understood it well. But I guess that Set Life will just work. Well, thanks anyway :D

Thanks for your time
Dark Dragon

E- Spell being cast yadayadayada
C- Spell equals to yadayadayada
A- Unit - Set life of (Triggering unit) to ((Life of (Triggering unit)) - 50.00)


this would work... ;D