I have gone through divorcing a narcissist husband, more than once.
Narcissists usually take your trust and run with it, not letting their narcissism run rampant until sometime after you have married them. Then, you find yourself in a shitty situation, with a shitty person, often blaming yourself for getting there. So, one thing is to be sure you are ready, legally, emotionally and mentally for the barrage of shit you will undoubtedly receive from this woman you want nothing more from than to be apart and never aware of How to divorce a narcissist wife existence again.
She will, fling as much shit at you and about you as she can summon up.
None of it needs to have even the slightest hint of reality, just so long as she believes it will diminish you so she can win. Prepare yourself for more irrational behavior than ever before, for her hateful insults and attitude to multiply like cancer cells, for you being held responsible for anything and everything bad she can possibly assign to you. If you have not yet broken to her your intent to divorce, get all your thoughts in order for when you do, and do it when you are feeling secure and immovable yourself.
If you are leaving your marital home it will likely be easier, because she cannot legally, and hopefully not physically, stop you.
She will try, probably. Any way she can. If she is the one who has to leave, get everything you possibly can for the divorce done before telling her. I know this may seem like you are living a deceit, but it is really self-preservation.
If you are in the US, file divorce action and have her served while she is somewhere other than your home. If she needs to be removed, try to expect the unexpected. I had to have the police come and physically remove one man to get him out of the house, and the threat of a personal protection order kept him from causing me harm or returning.
Fortunately, I do not freeze when looking up the barrel a gun, and I just walked to the phone and called the police. They came and he had a big shit fit in front of them. They were only too happy to haul him off and charge him for his offenses.
She may go either way or any other way. Be ready for her to try anything to get you to change your mind. Stand immovable in your resolve.
Also, expect to lose some of your material items to her, because she will do her best to take what she thinks you most value. She will not be reasonable, but she will think she is. He really believed that was a possibility even as he screamed nasty things to and about me and threatened to take everything, including our son. Fortunately, I was the How to divorce a narcissist wife and I only lost half of everything we had.
I say fortunately in jest, sadly. I wish you the best in your quest for freedom. Please understand I have told of my experiences only because I know how horrid it can be and want you to have an idea of what may come. Have a male friend around for support and eye witness, if possible. It can make a hell of a difference in your favor. You have my sympathy.
You need to remain firm on your decision to divorce and do not consult her on it or give her any opportunity to try and change your mind. Document everything and gather all the paperwork you need regarding finances etc. It will get nasty and you need to be prepared for false allegations and complete and utter downright lies.
Make sure you get exactly what you are entitled to and a fair settlement. She will no doubt lay "How to divorce a narcissist wife" the sob stories and go in to full victim mode but do not fall for it. Remain calm and non emotional in any dealings with her and do not give her any kind of reaction as this will be used against you and she will push every button you have to try and reinforce her false stories that you are the problem.