You've already had the excellent advice of making sure you tell the police, school, work, bank etc Just make sure that the police also know your new address, and that you are noted as 'at risk', so that they would come quickly if called.
I'd also encourage you to consider taking the kids out of school for a while, as he will know that he can find you there at drop off/pick up etc and follow you to your new house. (Also a few days off work if you think he'll show up there.)
Also consider whether he could find your address by looking up credit score companies and looking under 'linked addresses', as you will be 'linked' financially, so any new applications for utilities etc will be listed, along with your new address. (I'm not sure what you can do to prevent this, but maybe someone who's done it could advise you here ?)
Also recommend you join a credit score company too, in case he tries to open up credit in your name. This isn't unusual, because they know all your details/ signature etc and they always try to get 'revenge' because you've dared to defy him.
Also make sure any family/friends etc are aware, and that they are not to give out any information at all if he phones or visits.
Cancel your mobile contract or ask for a change of telephone number. ( I'd actually also do a 'factory reset' on my mobile in case there's any tracking apps etc that he could use to find you too.) Keep all communication to /from him via email ( Open a new email address specifically for him and block him from your usual email. Also remember to change all passwords for everything when you leave) because
- You can take your time to compose your message, and he cannot interrupt or talk over you etc so he'll actually have to read what you write.
- You can turn off notifications and only log in to read any messages when you feel able/ up to it, so he can't 'bully' you by forcing you to listen to him.
- You have proof i.e a written log of all communication, including any and all threats/ bullying/ intimidation etc, so he's less likely to keep up the 'bullying' behavior in writing.
As for the 'letter', I'd keep it factual e.g
'This marriage is not working, and I've decided it's time for us to part ways. I will be filing for divorce. My mobile number will be disconnected, but you can email me at ................... if you need to discuss arrangements for the children.
Above all, keep yourself and your DC safe. Don't be tempted to 'relent' on anything to help him, no matter what sob story he tries to spin you, because it will be lies, designed to get access to you, because he can't bully you if he can't get to you.
Never allow him into your new home, ever, even if your DC are crying and he's outside pleading with you, because the moment you relent, he'll become a monster again.