An emphatic expression meaning 'none other than' or 'exactly (this person/thing)'; used to single out and stress the subject or cause.
この成功は他ならぬ君の努力のおかげだ。
This success is none other than thanks to your effort.
他ならぬ私がまずお詫び申し上げます。
It is I, none other, who must first offer an apology.
犯人は他ならぬ彼自身だったと判明した。
It turned out that the culprit was none other than himself.