You may have seen stories of parents who split up but are still able to work as a team, and even include new spouses, to raise the children they share. While this might look like a storybook outcome for a child custody arrangement, it isn't one that's possible in all cases. For a person who has an ex who's a narcissist, trying to co-parent would likely be an immensely frustrating venture.
A true narcissist won't ever be able to accept not having things their way. If they're forced to deal with not getting what they want, they may begin to act like a petulant child. This can manifest as anger, but it may lead to passive aggressive behaviors.
Because of the mental state of the narcissist, co-parenting typically won't work. They don't have the ability to compromise, and they can't stand having another person to make decisions with. Even if your ex claims they want co-parenting to work, it likely won't without them receiving intense therapy.
In order to make any sort of parenting relationship work with a person who's a narcissist, you have to be able to set firm boundaries. This includes for factors like communication and parenting decisions, but also for your emotional and mental health. You must determine that you won't let their antics get to you because they are going to try to get under your skin.
Another problem that comes up that your child may see the interactions and suffer ill effects. While you shouldn't feel sorry for them because of the things their other parent does, you should help them find ways to cope with the actions.
Having a solid parenting plan is necessary when your ex is narcissistic. Make sure that everything is clearly worded to minimize the chance of misunderstandings.