How do you mean "Bag system" ? :S
[:thup:] You want an entire new inventory for the unit ? :S
[:thdown:]Or just make him unable to pick up certain item-types ?
Look this trigger:
Events
Unit - A unit Starts the effect of an ability
Conditions
(Ability being cast) Equal to Atacar
Actions
Unit - Cause (Triggering unit) to damage (Target Unit Of Ability Being Cast), dealing (3.00 x (Real((Strength of (Casting...