Vrock
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As I understand it, the problem lies with the Dominion ring (a modified artifact called "Ring of the Shadow-Stained"). Here's its modification:
This means the ring adds 65 to attack, 45 to defense, 65 to magic power, and 35 to knowledge, as well as 2 to morale and 3 to luck. This is what allows you to win the final battle. If this bonus doesn't occur, it means that there is something in the Maps, UserCampaigns, or some global mods that conflicts with this map.
I solved the problem by temporarily deleting all maps from the Maps folder. I loaded my save and the ring acquired the correct stats.
But I replayed the battle several times and eventually won! I feel much better now.
Thank you very much for the great campaign!