Dirty_Player
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I used to think that the unicorn's aura gives 5%, but it turns out it's 20%. Regarding the summation. If the resistance of dwarves (20%) and unicorns works as a sum, then the resistance will be 40% (0.2 + 0.2 = 0.4), and if each works separately, then 36% (1 - 0.8 * 0.8 = 0.36). The difference, in theory, is not significant.
To find out the truth, you need to test a high-level Thorgrym and give him 100% resistance (for example, Battle Dwarves 40% + Resistance Expert 15% + resistance artifacts 15% + the necessary level). If they are summed, then you can bombard him with Arcomage for 100 rounds, and everyone will remain alive, but if they are multiplied, then it will not be possible to achieve 100% resistance, and someone will die.
In short, I don't have reliable information on this issue. Someone tested it, and they say that each works separately, but I don't really believe it.