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I suggested this option to you (but there's no talk of the developers implementing it): two units, without upgrades, but alternative ones. Then it would be possible to recruit succubi or incubi. But in the "upgraded-unupgraded" system, they just don't fit.
I really liked the imps in "Heroes 5". They're so cute :). I'd like to see them in "Heroes 6" too.
Well, if the unit is the same, then the name isn't that important. There have been many discussions here about how there are many angels, but only one devil, so he can't be made into a unit (in the Bible, the devil is one - the lord of hell). The main thing is that the unit exists and is recognizable, and the name is a secondary matter.