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Perhaps the Dark Champion could take the ability that reduces morale by -1 (I forgot what it's called, something about a frightening gaze) and pass it on to the Alt, plus, as you said, the fear attack.
And give the Dark Champion vampirism, like the Archangel Alt, and leave the forgetfulness ability?
And increase the HP of both, but not excessively.
As a result, we'll get logically sound skills for the units.
Dark Champions with regeneration and mind-affecting abilities because, let's say, they are vampires. And Phantom Dragons with an emphasis on intimidation because they are phantoms.
Here, too, the main thing is not to overdo it...