I'm not sure if these two have the same meaning, but I don't think they bear the meaning that I know which is the basic なる (to become).
内なる声と言いますか・・
彼女よりさらに深遠の、未知なる人物こそが黒幕なのではないかと疑わせるのだ…。
This なる is a form of the old copula なり (equivalent to modern だ), and specifically an earlier form of the usage that became the modern な particle used with adjectives. There are a few adjectives that can still take なる instead of な in modern Japanese; this form generally has a literary or poetic-sounding effect.
In the case of the specific two adjectives used here, 内 and 未知, the なる form is actually the only form they can take as な-adjectives in modern Japanese - 内なXX and 未知なXX aren't really used. However, they can be used regularly as noun forms, so 未知なるXX could be rephrased as the equivalent but less poetic-sounding 未知のXX, and 内なるXX similarly as 内のXX (though in the latter case it's perhaps not the most natural expression).
It's the 連体形 of the old auxiliary なり. You can replace it by である.
なる has several different meanings in Japanese. "To become" is just one of them.
From this dictionary, there are the following meanings of it:
なる
〔断定の助動詞「なり」の連体形〕
① …にある。 「内－世界」
② …という名の。 「顔回{がんかい}－者」
③ …に当たる。…である。 「義兄－人物」
Your sentence should fall into this category.
I would interpret “1.内なる声” as “my inner voice,” and 2.” 未知なる人物” as “the person I don’t know yet.” “なる” used in both examples means “which / who is,” like the voice which is (sounds) in my heart, and the person whom I’ve not known / met yet.
We can say：
複雑なる関係 – relationship which is complicated
彼の妻なる女性 – a woman who is said to be his wife.
日本のピカソなる棟方志功 - Munakata Shiko who is regarded as Japanese Picasso
更なる投資 – further investment
All different from another usage of ….になる、meaning to become.
"ABC なる xyz" is a pretty common usage, and not an old locution.
