Exposing a Narcissist. Exposing my Ex.

I was in a relationship with my ex for 6yrs. We have a daughter. For the entirety of the relationship I was put through a barrage of Narcissistic abuse.
He has a Narcissistic Personality Disorder and is a Sociopath. It’s taken me a few years to realise what he is and what he’s done to me but I’m now beginning to understand with the help of fellow victims, counselling and lots of books and online blogs.
During the relationship he was Mentally, Emotionally, Physically and sexually abusive and every time he was he’s blame it on me. He ground me into the ground. He walked out on me so many times and always came back. Always leading me to believe I was worthless and would never get anyone else because I was a Fat, Ugly Mess (his exact words). So every time he left I just sat and waited for him to come back. And he always did.
He discarded me in May this yr. And within days was in a new relationship. That relationship lasted around 6weeks and he messaged me wanting to come see me and sort things out and of course I let him because I was so broken by everything he’d done to me. That was July, and since then, he’s come and gone 5 times. The most recent being 9th to the 17th November. He left me the day before my Fathers Funeral. It was awful. And I’ve just today realised that every time he left me in them 6months he was going back the woman he left me for in May. He was going between us both. Playing us both. Lying to us both. Today I told him that I know what he’s been doing and I threatened to expose him. To tell her exactly what he’d been doing. And he started being abusive and threatening me, and still now nearly 12hrs later he’s messaging me. He’s threatened to do things to me that will cause a lot of upset and trouble if I tell this woman what he’s been doing. I’ve told him I’m not scared of him and to do it and he’s started back peddling. I’ve told him to sort out contact with his child trough a solicitor as I don’t want to have to see him anymore and he’s not listening, he’s constantly asking me when he can take the kids out. The same Kids he completely forgets exist when he’s not here. The same Kids he hasn’t paid a penny for to 6months. The same kids that one is now in therapy because of abandonment issues caused by my ex and the other not knowing who he is because he’s not been in her life. Yet he makes out to himself and to her that he’s this amazing Dad. That I’m the one that’s bad and it’s all my fault that he doesn’t see the kids.
It’s unbelievably draining having to deal with this man on a near enough daily basis. I don’t know why I’m saying all this. Just to get it out I suppose because keeping it all in makes me crazy. I really want to tell the woman what he’s been doing but I know him, he’ll worm his was out of it and make me look like I’m a crazy bitter jealous ex. He did it with me when a woman he left me for in 2015 contacted me about everything he’d done and said. He denied it all and made her out to be the mad one, even though I had all the proof. I still believed him though. Thinking about it now is utter madness. But he’ll do the same to this women even though I have all the messages, photos and videos to prove what he’s been doing. I wish people like him didn’t exist. It’s beyond cruel to treat people the way he has.