@Hilfemich
One thing to remember is that he will say anything to frighten and intimidate you to 'get you back in your box'. So his "I will take the kids" or that he 'won't let' you leave with the children aren't necessarily true. But you are his 'cash cow' so he's going to say and do anything in order to keep you down.
Granted if you tried to walk out the door in front of him with the DC he'd raise holy hell, but only to upset the DC, upset you, and then blame it on you. But should you leave with the DC when he is not home (I am NOT saying to do this) nothing short of a court order would force you to hand them back over. Granted, the same goes for him with you. Were he to take the DC and move out you'd have to go to court to get access.
As far as his "I will get custody of the kids" that's just bluster. No court is going to take your children away from you simply because he says for them to do so. Yes, he'd have access unless he's a clear danger to them, maybe even 50/50 residency but that's a decision for the courts to make.
This is where I was saying that you think he has more power than he does. And why it's imperative that you see a solicitor. You desperately NEED to know the actual legal truth of your particular situation. All of us can give you our opinions and/or knowledge of the law from a 'civilian' perspective, but only a solicitor can truly set your mind at rest.
I don't mean to cause alarm, but you mention his family have 'unwestern' ideas. Does this mean they/some of them live in another country? If so, do you have any concern about this shithead trying to take the DC there? If so, you need to alert passport control so their passports can be flagged.
And as PP said, you need to be SILENT about any plans or solicitor visits. Say nothing to him. Don't make 'threats', don't try to 'talk to him' or 'make him see reason'. In fact, avoid talking to him as much as possible.