The phrase "that now prevail" functions as a relative clause modifying a noun or noun phrase in the context of a sentence. However, without the complete sentence or more context, it is difficult to determine the exact function of this phrase.
Relative clauses provide additional information about a noun or noun phrase and are introduced by relative pronouns (such as "that," "which," or "who") or relative adverbs (such as "where" or "when"). The function of a relative clause can vary depending on its role in the sentence, such as acting as an adjective, providing additional details, or specifying a particular subset of the noun or noun phrase.
To provide a more specific analysis of the function of "that now prevail," it would be necessary to see the complete sentence or surrounding context in which it appears.