Silence settled. Billy stopped groaning and dropped off to sleep. Nelly stopped knocking on the clock door and dozed. The fire in the grate hushed down into ashes.
I read the above in my son's book.
What is the grammatical form and function of "down" and "into" in "hushed down into"?

"down" — adverb. With "hush" it creates a phrasal verb "hush down".
"into" — preposition. It indicates transformation, as in "changed into (something)".