Hopefully you can afford good legal counsel. Search "high conflict" + "attorney" and that should give you some results of attorneys that have experience litigating against Cluster B personalities.
Honestly, going to court with a mental health diagnosis is not as sound a strategy as going to court with documentation of behavior and actions along with documentation of what the results of the behavior and actions brought about ("proof of damages").
The judge won’t really care if the person destroyed property and made threats because they are NPD, BPD, Histrionic, or just an a-hole. The legal system wants proof of damage in order to prosecute the person who did the damage. They don’t necessarily care why the person did it, only that they did something and what the damage was, how much it cost to fix, etc.
So, if you are in a state where one party recording is admissible, use recordings. You may also use photographs, threatening (or bragging) emails the narcissist may have sent, letters, testimony from mutual acquaintances, etc. The narcissist generally believes they are smarter than everyone, and that they are above the law. It’s not very hard to get them to brag about their illegal activities, their bullying, their attempts to send flying monkeys to bully, harass, or physically threaten or attack people, etc. The legal system will be very interested in that type of evidence. You should not need a mental health diagnosis to obtain a restraining order or perhaps some jail time for the narcissistic abuser.
*Disclaimer: I am NOT a lawyer! Consult a lawyer!*
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