English morphological rules detail how words relate to other words. Thus, my best assumption would be because BOLDEST is an adjective whereas BOLDLY is an adverb.
Everything you need to make a good meaningful sentence (and grammatically correct, of course), is to follow common rules. Do not forget to provide your sentence with complexity, but you don't have to make them too complicated, I mean you need to use key words, they will make your thought completed. And the second point is that you need to properly organize the sentence using correct word order. ion:
I believe it’s mythical because a myth is an old often historical story
