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.