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The race is flexible, there's no arguing that. But it's VERY demanding in terms of skillful play and very sensitive to losses in battles with neutral creatures, which is why starting as Demons is often quite difficult. Therefore, in the hands of a novice, a Demon is the least dangerous opponent because they will likely be defeated before even reaching enemy territory.
In terms of gameplay, Demons simply enslave, because without mana, they are not very effective. It's more complicated with Necropolis and Academy, because it's unlikely that you'll be able to drain all the mana from a Mage, and a Necromancer can restore it with a spiritual connection. In battles with warrior races, you'll have to rely more on magic, otherwise the army will be very weak, and Haven, Light Elf, and Stronghold have very powerful attacks. But the main principle of the Demon is a quick offensive and no defense; in prolonged battles, the Inferno army performs poorly compared to others, unless there is a significant overall advantage.