A narcissist is a person who doesn't feel good about themselves. In my opinion that is the basic issue one has to deal with when in a relationship or attempting to leave a relationship with a narcissist.
In order for the narcissist to feel good, he/she must make you feel inferior. During divorce the narcissist will threaten you with things they are most afraid of. For example, if the narcissist depends on his reputation to feed his/her need to feel like a good person, he/she will threaten to ruin your reputation. To alleviate their fears they project them off on to you.
How do you stay sane when divorcing a narcissist? You keep in mind that the narcissist has low self-esteem, they feel inferior to you and out of control. Their abusive behavior is not about you but about emotional survival for them. That is why it is important for you to protect your own emotional well-being before, during and after the divorce process.