Item Abilities - Passive Attacks

[Emigliah]

I am creating a map which requires a type of item that attacks when in a unit's inventory, without the unit having to do anything. The best example of this is Battle Ships. Does anyone know how this was done?

 

[Panto]

This question is practically deserving of being in some kind of FAQ.

Give the item a Phoenix Fire-based ability, or make a modelless invulnerable dummy attack unit and move it to the position of the item-possessing unit every small interval of time.