Nargott
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maybe, like luck, but the probability of it triggering won't be more than 50% (neither for Defense Bonus nor for luck, meaning the maximum probability is 50%), this was done after one of the expansions. For example, you have Expert Leadership and an entire human army - DB=8, and the opponent reduces it by 4 (e.g., Banshee's Wail from a Dread Knight and Cloak of Shadows) - the result is your DB=4, not 1.
I've tested it, and DB, like luck, can be displayed below -5, but the actual value remains -5, and it is this value that is used when calculating the damage. So, 1 Banshee can never deal more than 150 damage to a single target with its Wail.