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Well, I don't know, I don't know, but look, I played the fifth mission today, captured Shahbad, and Galib, even with my "Strength over Magic" perk, still had 17 points in magic. My Gotay has the "Barbarian Luck" perk, which gives him +5 luck. So, his Fireball spell dealt about 80 damage to my troops, and 240 to the tent, which means it does work.
PS I remembered that Gotay had a Dwarven set of two out of four artifacts, which improves magic resistance by 40%.
Just for fun, I'll try replaying the battle without this set.
Added 1 hour 24 minutes ago
Well, that's it, I tore Alaric to shreds! Losses were about a hundred centaurs in total. And Gotay's stats are just insane, he's a terminator, even Freya didn't have such stats when I played as the Lords of the North. Attack is over 50, defense is over 50, knowledge and magic power are over 30. By the way, I took almost 3,000 Magnetic Golems for the final battle, but I didn't even have time to get them into the fight, I solved everything before that.
PS Before the last battle, I trained Gotay in Education instead of Machines, because the "Wrath of the Horde" spell didn't drop for the Kujin, but Gotay has it, and he will have to teach it to him, if he can.