I am unemployed because a previous employer sent a letter that alleges slanderous accusations; what are my rights?

In the state of California, this sounds like a violation of the Labor board.  Why would an ex-employer voluntarily send a letter like that unless it was requested by your new employer.  And even then, you would have to sign a release allowing your new employer to question ex-employers.  You don't say whether you are employed by a public entity, which would make a difference.  Please contact me to discuss further.  deborah.barron@lawbarron.com              

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If you believe the accusations in the letter are provably false (or more specifically, that your former employer cannot prove them true), then they committed defamation against you which resulted in substantial economic loss (loss of a job)--you could sue them.

If you believe that you cannot sue, because the accusations are  fundamentally true, it was still inappropriate for a former employer to share with with anyone; it may not be defamation, but it is still interefernce with a "contractual" (employee-employer) relationship. (There could still be liability, but it's less clean cut than if there's obvious defamation.) You should indeed at a minimum send them a letter demanding they cease doing this and letting them know that you will take action if they do so again.

In either event, you should consult with an employment law attorney with whom you can share all the details in confidence and who can advise you on the best course of action and help you carry it out.

If you were threatened with a gun and suffered mental distress because of it, btw, you might also consider a lawsuit against the "scary new boyfriend."