[Executor]

There's just this,
A horror (lesser) isn't consider strong enough to make a mage cast swarm.
Swarm flyes are magic beings, they don't have magical attack.
And, IMO, horrors are used to take out 1-15PD provinces, with NO mages.
If you script your mages for horrors and something else comes along, well not good.
And if you have enough mages, horror won't do much damage anyway.

And if you're looking for a way to stop horror+ scout taking your provinces, it's a bit expensive but worth it sometimes. Have indie commander (able to kill a scout) roam around all, ALL, your provinces in circles so they retake a province after the horror takes it. The horrors will stop eventualy, if not even better since your enemy is wasting blood slaves.