I couldn't see any effects after casting it a few times so I tried testing it. First I cast it at a province with an enemy army that I had a scout in. No battle was reported and he enemy army was fine. Then I cast it at one of my own provinces(with an army.)
I have had a "Send Horror" work on those with Horror mark on in the province. Sort of like a limited Vengence of the Dead. Come to think of it; i've only cast it a total of 2 or 3 times. And by the description I suppose it's to work like any other artillery spell.
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Great indebtedness does not make men grateful, but vengeful; and if a little charity is not forgotten, it turns into a gnawing worm.
I also had the same experience with the 'Lesser Horror' spell.
PLUS I've had the same problem with the air spell called 'Phantasmal Attack'. I casted it on an independent province and a computer controlled province... yet both times NOTHING.
The next turn says the spell was casted... yet no battles or anything. Hopefully this gets fixed.