"I have to grin and bear" — I don't believe in that. You need to speak up and let your issue to be known. "the boss doesn't care" — make them care. Or at least, "put the ball in their court". Bottom of the story, you don't have to "bear and grin". That's what you brought here. You need to start making the steps leading you out of this. You may rely on others' help. But those others capable of helping rely on your first step.

If you have not invited her, have not spoken to her or to any of her team about it, and don't want her there, how do you know she will find out about it, and, furthermore, invite herself to attend? That seems like quite the stretch of the imagination; does she have a history of this?

Also, with someone who caused you PTSD, consider you might not have to seek polite options. As seen from @sf02's answer, what you might need from now on is: firm and just. That's all.

You think she is just going to show up, with a brood of interns in tow? Did you post a flyer in the break room with your home address?

@ColleenV I feel OP might have meant how to avoid triggering retaliation. (Which, to clarify, is a pretty hopeless endeavour; the bully's power is in retaliation; I indeed agree that the way out is to ignore that.)

This will not end well for your career, but assuming you are in a locale with strong rule of law and property rights, the legal system could be available. AFAIK, trespass is both a civil and possibly criminal issue

@user126509, I am curious about how that coworker knows about your party ? I assume you only send the invites to your boss and your interns. So, when you send the invites to these people, perhaps you can add a BIG note: "Please do not forward this invite to anyone else because ONLY the people on the mailing list are invited". Does it work out that way for you ?

I cannot come up with a full answer but two things are clear to me. 1. Get designated help. Assign someone to confront her for you when the time comes, or even to explicitly tel her or her interns that this is a team-event. 2. Try to preempt this confrontation (But I don't know how).

@Job_September_2020 some people are just nosy and will respect no boundaries because they think they are entitled to something. They usually have their ways of getting information like that (manipulating, eaves dropping, etc..)

Sorry about the PTSD, my wife suffers from it and it's got so much more power than people realise. Particularly with the source of it around you at work. It is possible that they won't find out and turn up, but it sounds like what you need most of all is backup. Your home has to be a safe place. Is there a friend or two you can trust to get in the way if she shows up unannounced? You're fearing possible confrontation, so the right answer may be to have someone else to confront her for you. Even if she doesn't show up, you'll feel safer. (I've done this for my wife a number of times).

@Levente Please post your answers as answers. Disagreeing with the question won't help anyone without a solution.

@Mast I currently don't see how I could top sf02's answer in any ways. The core of it is now written down. Beyond that, I just wanted to help OP. (But those things do not technically answer the question, so may be less justified to be the content of an "answer".)

What country is this in?

What kind of party is that? Is this meant to be half-official company party on your premises, or it's a birthday party where you invite most of your interns? In both cases, lowering the boundaries between your private and company life might not be a good idea, considering PTSD.