“Heroes of Might and Magic V: Tribes of the East”

rather, the opposite: immunity will appear with the same probability as different schools of magic in other factions - i.e., often. Otherwise - how will the remaining slots be filled?

well, let's look at the magic drop rates for other factions:
class light dark chaos summon
knight 8% 8% 2% 2%
demon 2% 8% 10% 2%
ranger 8% 2% 8% 2%
warlock 2% 2% 15% 10%
mage 10% 8% 10% 15%
necromancer 2% 15% 8% 10%
runic mage 8% 2% 10% 4%
from this, it follows that the probability of a barbarian getting the skill will be about 5%.