You cannot not talk about it; not if you want to be hired.
A blank space means one of two things to an interviewer (if you even get that far):
- You were unemployed
- You were in prison
So unless —as some of the comments are suggesting— you did work in organised crime, I'd seriously question the enforceability of this exit NDA. Trying to gag somebody after they've started is pretty stupid. That's why most companies have you sign them before they show you the secret stuff.
Objectively speaking, leaving this NDA hanging over your head unresolved is probably going to affect your future employment. It's a question-mark on your record. A candidate without a dubious employment history is probably going to get your job.
So try to get rid of it. Talk to your past employer and come to an agreement. Don't sign anything else though. If they want you to sign more rubbish, talk to an employment lawyer first. If they don't want to drop it, at least come to an agreement about what you can say.
Failing that describe what you did, the skills you used, how you developed, etc... But expect to answer questions on it and expect to discuss this non-disclosure settlement you've slipped into.