Spell help

[Knightmawk]

I want to make a spell, similar to dark arrow, that spawns a unit from a killed one. Except in this case I don't want to make it a spell that autocasts, I want to make it an ability that has a percentage of happening, like has a 15% chance to spawn unit x from killed unit.

Is there a way I can do this without triggers? and if not, how do I do it with triggers?

 

[Parrothead]

This would have to be made with triggers. I'll create a custom map of the spell. Give me a few minutes.

Edit 1:

 

[terradont]

It does not have to be made with triggers at all, at least not for ranged units.

Base it of Orb of Lightning(new), set percentage to hit, and change the spell to a Black arrow with 0.01 duration and no mana cost or cooldown.