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In Heroes 3, there's a missing feature from Heroes 5: when you upgrade a spell, besides the example of gaining an "expertise" slow effect, the base effect remains... which would be very helpful for encounters where you don't want to reveal a fully upgraded spell but still want to apply a slow effect...
"Бороться, бороться, пока не покинет надежда, что может быть в жизни прекрасней подобной игры." В.Каверин "Два капитана".