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I even made a mistake; it's not -22, but -23.
Here's the calculation:
-8 from the banshee's scream (-6 super abilities, -1 from the dreadnought's specialty, and -1 from the set of necromantic artifacts);
-6 from artifacts (2 rings and a cloak -2 luck and defense);
-4 from sorrow;
-5 from creature abilities (-1 phantom dragons, -1 poison aura, and -3 from nightmares);
In total, it's -23, and if we exclude the nightmares, it's -20, which is also significant.
However, the knight's morale bonuses won't be able to overcome this, as the maximum bonus can only be +15: +7 from artifacts (lion set, sandals of the saint, and armor of valor), +3 leadership, +2 prayer (if Godric), +2 bonus if the creatures are of the same race, +1 from archangels.