A few questions:
 

1. When a horror attacks a mage under a Dome of Corruption, is it guaranteed to
pick your horror-magnet (a commander who has been hugely horror marked)?

2. Is Strands of Arcane Power limited to friendly dominion? How is the struggle
between the caster and discovered astral mages resolved? As a mind duel, but
with feeblemind instead of death? Are non-astral mages safe as long as they can
sneer at a mind burn? Do penetration items on the casting mage help?

Re: A few questions:
 

1) I expect that it could attack any horrormarked mage, but odds are the magnet would get it just because he has a lesser chance of failing to trigger an attack.

2) Yes. Seems like a magic duel with feeblemind for loser. I expect penetration items will help with the mind burn, but not with magic duel.