He is threatening to do you so much damage that you can never, ever recover from it.
The phrase has been used about Putin bombing cities in Ukraine back to the dark ages (Dark Ages meaning Medieval times, before there was running water, electricity, cars - people travelled by donkey and lived in huts). It means total and utter devastation. Think of the images of Hiroshima after the nuclear bomb.
If he had any violent tendencies I'd be worried he was going to put me in hospital or try to kill me. Burn my home. Cut brake cables on my car. Pay someone to throw acid on me aka Billie Piper's ex.
If he prefers emotional/psychological abuse, then I'd be worried he would do something to severely harm my character to the point it jeopardises my work and/or ability to be social. So making false claims of child sexual and or physical abuse about me to the police. Or email my boss with extreme lies about me - I'd been stealing from work, something made up sexually about me. Or email everybody I know with something similar. Tell people I have a severe mental illness, or imply that I do. Or that I'm a compulsive liar. The point being he's out to destroy in a way that there's never, ever a real way back from. Total destruction of your life is what the threat is.
In both cases I'd be afraid he would harm my children.
The nuclear option is the bomb to end all bombs. Destroy civilisation. Saying it to a person is the threat to end all threats. You cannot actually make a bigger threat than that. And it wasn't a mistake because he repeated. It should be taken very seriously, because it is serious in the most awful way.
If this was me in the UK I'd go to the police about the two threats alone, because I would be very afraid. Ir, to protect myself and my children, I'd stay away from them to protect them.