Used with a negative verb to express that something has never happened or is not at all the case. Emphatic and somewhat literary.
I have never heard from him.
I have never heard such a story.
Usually written in kana; the kanji form 終ぞ is rare.
Rare kanji spelling; learners should use the kana form.
The origin is uncertain; it may be a contraction of an older emphatic expression. The kanji 終 (end) is likely an ateji for the sound.