Divorce is complex even when your ex is mentally stable. When you're dealing with a person who's a narcissist, things can get even more difficult. It is imperative that you prepare yourself for this when you're going through the end of your marriage because they are going to try everything they can to get their way.
A narcissist isn't going to play fair. Instead, they are going to try to bully you into giving them their way. While they're doing that, they may try to make you as miserable as possible. For example, they may try to force the divorce to go through a trial instead of using mediation. Not only is this an attempt to drag you through a longer divorce, it is also an attempt to make you spend more money since litigation is more expensive.
Even if your ex agrees to go through mediation with you, there is an excellent chance that it isn't going to work unless you give in to all their demands. Negotiating with a person who's narcissistic doesn't usually end well because they don't have the capabilities to realize that they can't always have their way.
During the divorce, they are going to make it seem like you're the one to blame for everything. They can't handle accepting the blame for anything. Because of this, you're going to need to shore up your mental barrier so that they don't cause you to break down.
Remember, you have to protect yourself throughout the divorce. This might mean that you have to take a step back to gather yourself and think about what's in your best interests. Your divorce team should be available to help you throughout the process.