Who would win? The shadowy Nightmare Forces or the powerful Dark Spirits?

1920127
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This is a good question. However, the two factions are just so utterly different in how they interfere with others. The Ni-
Oh, great, I forgot, I have a dilemma here. Are the Nightmare Forces the Moon animals corrupted by a singular Nightmare Moon spirit, or are the Nightmare Forces a group of spirits, amongst them that of Nightmare Moon, that corrupted the Moon animals?
If the former, then probably the Avatar spirits, from sheer power and speed. If the latter, then it all depends on the entities taken over by the Nightmare Forces

1920824 Like you said, the Nightmare Forces are moon creatures possessed by the Nightmare Entity while the Dark Spirits who, while not entirely evil, are too angry to even think and act as savage animals. Think the demons from Princess Mononoke.