Is roared a mighty roar a adverbials?

No, "roared a mighty roar" does not contain adverbials. In this phrase, "roared" is the verb, "a" is an article, and "mighty roar" is a noun phrase acting as the object of the verb. Adverbials typically modify verbs, adjectives, or other adverbs, providing additional information about how, when, where, or to what extent something occurs.