The general results of Astral Corruption was that it would attack any mage who was forging or casting a ritual spell with what seemed about a 20-30% chance. You would always get your spell or item off (i believe?) but sometimes the mage would be attacked.
As for my experience with AC personally: it was a big problem for me. It hit right at the end of the CTis war and prevented me from summoning, which was the only way I had of producing elite troops! Unsurprisingly that war ran reallly reallllllly long as a result. After that I pretty much just had to put a hold on summoning and concentrated on battlefield-only spell strategies which did not rely on month rituals.
Later tho, I learned from Sensori that Astral mages have a fool-proof loop hole in Astral Corruption. They can just cast Returning at the start of the horror attack and escape immediately. After I learned this, all my magical skills instantly came back online without any real problems (except needing to move mages out of my cap after Returning, now and then). This is in fact the real reason I stopped the 2nd attempt to Dispel AC that I started
I probably could have dispelled it as I had a large cache of pearls and recieved 60 or so more from other nations. But with my loophole in place, AC was hurting all other nations more than my own!
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P.S.
I didnt notice anything about these horror seeds you mention? All I saw was horror attacks on mages doing rituals/forging. Additionally, I noticed no effects from Illwinter - but I think I was too far away for your dominion to effect me.