Units attack spawns a unit

[Kabel]

When one of my ranged units attacks an enemy unit, a buff is added to that unit that lasts 3 seconds. If the enemy is killed while it has the buff on it, a friendly unit shall spawn from it. Just like how the ability Black Arrow works.

But I do not want the unit to have it as an ability that needs to be activated.
I want it as a passive ability that is always on.

Can this be done by modifing any of the abilities in the object editor or by triggers?

 

[Kercyn]

Hm hm, you can't check if the dying unit has a buff on it, so that is out of the question. Try creating an ability based on Black Arrow, make it autocast via triggers and hide its icon.

 

[Shdow89]

It can be done via trigger or object editor. Using object editor is much more simple. Find ability called Orb of Lightning(new), make it hero ability, put the chance on it on 100 %. Find black arrow ability, make copy them until you reach how much levels you want. Name it <Spell Name> level 1, or w/e you want, and reduce mana cast and cooldown to 0. Now in Orb of Lightning ability, find the where it says level 1 effect - add level 1 black arrow, in level 2 - level 2 black arrow, etc. You can choose in Black Arrow abilities, a missile you want to use, which unit you want to spawn, etc.
Oh, btw, you can make a lot of spells this way, as long as the effect spell is Single Target.