What’s the difference between the negative ず and the ぬ ending?

I’m not asking this for day-to-day usage, as I’m pretty sure it’s not used almost at all, but I’ve seen it pop up in media sometimes.

I cannot name examples, but I’m still curious, as it’s not cited in any of the sources I use (either that, or I hadn’t delved deep enough…)

Thanks for any responses!