Breaking up with a narcissist is like no break up you every experienced before. Your ex sees you as an object, their belonging, and this will drive them into a rage.
Here is what you can expect when you leave:
Stalking and harassment.
Devaluation and Jekyll and Hyde hoovering.
Smear campaigns and threats.
Remember, leaving an abusive relationship is better than staying. When leaving a narcissist, you need to plan (I didn't and am still dealing with it). Once you're gone, go "No Contact" or "Grey Rock" (if you have children together).
It is very important not to engage. Engaging a narcissist, even if it is defending yourself, is still feeding them their narcissistic supply. They want to know you are thinking of them and they are just as happy to have you fear them as they are to have you love them.