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The race doesn't change; the link to the damage effect changes.
But I looked at the Menad and the Nightmare.
In the original fear attacks, it's always Necromancy.
In the original poisons and crippling strikes, it's always Fresh and Blood.
Apparently, you can't combine them. If you remove fear or poison from the Knight, even without changing the damage effect, it seems to stop crashing (not precisely). It might even be that the damage effect has nothing to do with it (or maybe it does).
Can you check it?!
С уважением, Psatkha (Псатха) 
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Неудачное планирование ведёт к неудаче ...
NCF - мод, добавляющий дополнительных существ в HeroesV 3.1

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Неудачное планирование ведёт к неудаче ...
NCF - мод, добавляющий дополнительных существ в HeroesV 3.1