A Small Trigger Help

Ok i have a problem....Im making a spell called Summon Blood Elemental,The Spell Reduces Ur Hp By 20% Each level will incr the % By 5,The Only Problem..Sniff i dont know the triggers for it Som1 help me plz!!

Not sure exactly but could you set a event that everytime your heros max hp changes, it sets variable X = max hp * .2, then everytime that the spell is cast, it takes the casting unit and subtracts variable X from his current HP?

Hmmm I think That Would Work :) Anymore Ideas Guyz? ty ^_^

this is provided you've got all the capabilities to make your triggered spell

E: Unit Finishes casting an ability
C: Ability = Blood Elemental
A:
Set varUnitLifeLoss = triggering unit life total x (0.15 + [0.05 x spell level)
Set Unit Life = Unit life - varUnitLifeLoss

if you have never made a trigger spell before, you probably have no idea what your doing (unless you know JASS)....i suggest reading some of the tutorials, they're very helpful.