[Endoperez]

You could start it with a few castings of Astral Geysers, horror-marking enemy units.

I think horrors would, indeed, come. Phoenix Pyre, as an example, makes the caster explode and reincarnate upon death - but both caster and slaves are affected. Similarly, Hell Power gives the caster +2 magic and gives 20% chance of horror attack. I presume both effects also affect the slaves. They are part of the same spell. Otherwise, it'd be like Phoenix Pyre bringing the dead slaves back, without explosion or fatigue. You shouldn't get one without the other.