pre-noun adjectival
what a ...!; how ...!
Pre-noun adjectival used in exclamations to express strong surprise, anger, or admiration. Followed by a noun or noun phrase. Often translated as 'what (a) ...' or 'how ...'.
何たる幸運だ！
What luck!
何たる無礼な態度だろう。
What rude behavior!
expression
what ... is like; the nature of ...
Used in the pattern 〜の何たるか to ask or explain what something truly is, its essence or nature. Often found in formal or philosophical contexts.
愛の何たるかをまだ知らない。
I still don't know what love is.
彼は友情の何たるかを教えてくれた。
He taught me what friendship is all about.
何と is a more general exclamatory adverb ('how', 'what') that can modify adjectives and verbs, while 何たる is specifically a pre-noun adjectival used before nouns.
何という is a common phrase meaning 'what a ...' or 'what is called', used before nouns. 何たる is more literary and emphatic.
Contraction of 何 (nani, 'what') + たる (taru), the attributive form of the classical copula たり (tari). Literally 'what being', it developed into an emphatic pre-noun modifier.