psatkha
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I believe it wasn't created by the Heroes developers.
How do I create my own abilities, and WHERE (file location) should I create them? If they are script-based (so they don't only work on one map, but permanently on the creature)?
For example, I want to give the azure dragon a strengthened, chilling cold strike before attacking, and the aforementioned horses – one with a fireball after attacking, and the other with unholy word. How do I create an analogue of this ABILITY_BLADE_BARRIER?
You are mistaken... unfortunately, it is not possible to add your own abilities (at the moment), you can find a complete list of available abilities using the NOIADE utility (and I generally recommend that you study all the content of the first post in this thread)!
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