Now that the Americanon has given highly scholarly explanations, I don't have much to say, but I'd add a few additional comments that may be somewhat relevant to the original question >>214008471.

This phrase was recorded by a historian (司馬遷's 《史記》) as having been declared by the leader of the rebellion (陳勝, btw) when the Qin Dynasty in ancient China was overthrown (B.C.206) by the rebellious force.
This sentence symbolizes one of that greatest transformations of the continent, and its translation has long been referred to as a proverb in Japan. Some high school world history teachers instruct their students to memorize this. That is why classical grammar still remains.

>王侯諸将いずくんぞ種あら "ん" や
Even in modern Japanese, such "ん" appears here and there in a kind of fossilized form. They might trip up readers who have little knowledge of it.

For example,
>2023/04/30 — ここカイロの、あふれ"ん"ばかりの活気と人々の熱量を感じています。
>Here in Cairo, I feel an energy and enthusiasm that seems to be about to overflow. (= Cairo is overflowing with energy and enthusiasm.)

>2023/03/27 — 土下座せ"ん"勢いで拝み倒してくる鴨鹿を前に、猫山は溜息をつきました。
>Faced with Kamoshika, who was begging desperately and almost doing dogeza toward him, Nekoyama sighed.