This is by far the best thing you can say to a narcissist.

Narcissism is a tricky personality disorder: some people have it in a more intense form, others in a milder one. These individuals are full of themselves and always believe the blame lies with someone else. So what’s the best thing you can say to a narcissist?
Narcissism is a personality disorder characterized by an inflated sense of self-importance, a lack of empathy for others, and a constant need for admiration and validation. Interacting with a narcissist can be challenging, but there are strategies that can help reduce conflict and promote healthier relationships. One of the most powerful tools in dealing with narcissism is empathy, along with setting clear boundaries.
And this is the best thing to say: “I understand you.”
Acknowledge their feelings—but not their behavior. When communicating with a narcissist, it’s important to show empathy for their emotions, even if you disagree with what they’re doing. Try to understand and validate their feelings, but be clear that certain actions are not acceptable. You might say something like, “I understand that you’re frustrated, but I don’t feel comfortable with how you’re treating me.” This way, you show that you understand where they’re coming from, but still make it clear that their behavior crosses a line.
Don’t expect rainbows and cupcakes right away. Change takes time, and it may be a while before a narcissist is willing to adjust their behavior. But showing that you’re willing to meet them halfway—that you understand, that you get them, and that you can imagine how they think—is the first step toward building a better dynamic. It makes them feel heard and seen, and above all else, a narcissist wants to feel important. They always want to be right. If you acknowledge their emotions while not condoning their behavior, they may actually be able to work with that. It helps them separate their feelings from their actions.
By showing empathy, setting clear boundaries, and taking care of yourself, you can deal with narcissism more effectively and build healthier relationships—whether it’s with family, friends, colleagues, or romantic partners. It’s important to remember that your well-being should always come first, and that it’s okay to distance yourself from situations that are harmful to your mental and emotional health. And if you offer just a little understanding, you’ll often get something back in return. It’s at least worth a try.