Difficult subject... There are a lot of armchair psyche doctors out there, so be careful who you listen to.
This is a very awkward subject, especially for the narcissistic person involved who, like you say, does not believe they have anything wrong with them.
Dick, I was married to a severe narcissist for 34 years. It was a very horrible situation. She was, in addition to being a narcissist, a pathological liar. It was a very deadly ,caustic situation to live with.
It took me a lot of years to come to terms with this, after trying everything I could to make my marriage survive. Nothing works, nothing. The narcissist truly believes that they are perfect and everyone else has the problem. If you confront them on their lies and made up stories, they will dig in their heels and have a meltdown. The screaming, the swearing, the verbal abuse is unfathomable. They will stop at nothing to get their way.
And the narcissist does not care who they trample on or hurt to get their way. They put themselves above others and again, truly believe they are better, and the all knowing expert on any subject. They make up stories to make themselves look good. They lie, they cheat, and many of them steal. While they steal, they justify it by finding entitlement to what ever they take, using any reason.
Dick, I lived this for 34 years. It's bad, really bad.
I could go on and write a book about this. But somebody already did...
My best thoughts I can offer you is that there really is no cure. There is nothing you can do. I don't know how close this person is to you but you must protect yourself by not allowing them to in any way control you or get into your space. This is a very deep and complex thing. If you must interact with this person, keep it arms length. They will lie about you to everyone behind your back. I know this as fact from first hand experience.
This woman is the mother to my 4 wonderful children, and she was a darn good mother to our kids, really good. She has so many excellent qualities and traits and treated our children very well..... But the narcissist part created a lot of problems over the years in our family unit. I could go on for pages, but won't.
She even visits this forum to keep tabs on me, see what I'm doing, what I write on here. Unbelievable....
After 34 years, my children were adults, moved out and with lives of their own. After the patience of Job, and giving her more chances than I ever should have, I packed up my stuff, hooked up my TT and left. I filed for divorce soon after. Even that was living h... (hot place). Not her idea, therefore she had to fight me all the way on it. Sickening... Narcissists must have the upper hand, the final say, it has to be their idea.
There is an excellent book that came recommended to me. I bought it and read it. It has some incredible information in its pages and offers a lot if insight into this disorder. In that book are multiple quizzes you can print and check off yes/no answers to, with a score guide at the end.
Dangerous Personalities by Joe Navarro.
I bought it as an ebook for my iPad. I got the Kindle app to read it with.
I highly recommend you check this out, do a google search for this book. The book will tell you more than any of us here can.
Good luck, you'll need it!