Hmm... You ould try to use a dummy arrow spell which does nothing, and doesn't have a missile. If you'd want it to have fixed damage it would be very easy. You just give it to a dummy unit, which has specific damage, you create then the dummy unit at position of (Casting Unit) and order it to Attack (Target Unit of Ability being Cast). Obviously the dummy unit would have only the Orb of Corruption skill... What do you say?
No problem... Btw, don't be afraid to use Dummy Units as long as you don't want them for a long period of time. Also, I'll give you a tip: Try using the Expiration Timer trigger which adds a Timer to an unit, after which the unit disappears... This is very very cool because you no longer need to remove the unit after a bunch of waits, which in my opinion mess the spell up.
i want to make an arrow-ability that deals extra damage and decreases armor
This is very very cool because you no longer need to remove the unit after a bunch of waits, which in my opinion mess the spell up.