Events Unit - A unit is attacked Conditions ((Attacking unit) is A Hero)Equal toTrue ((Attacking unit) has an item of type ######)Equal toTrue
###### is your items type name. That should explain what you are after. Now uses | attacked unit | to refer to the unit that is under attack by the hero.
Warning If hero has area effect + that item ît may causes the game to crash attack if it's attack is not targeting a unit directly! (attacking ground)
Assuming you want what you mentioned in your thread title, Dr Super Good's method is not accurate.
Using an item is largely just like having whatever ability the item has on the hero that's using it. Therefore, make a trigger that detects when your item ability is used, as long as it's separate from any unit abilities.