How do I talk about my abusive former advisor if people ask?
First, let me say that I'm sorry to hear that your relationship with your original advisor has deteriorated to such a state that you have been emotionally scarred by the relationship. That is certainly not the desired outcome.
That said, you have a very important cautionary story to tell. Unlike cactus_pardner's answer, I don't think you should "jokingly defer," but instead take a very serious tone about the situation:
I wish I could be more positive about my relationship with Prof. X.
If you are serious about working with Prof. X, then there are some things that you should be aware of.
This may not happen to you, but I had some significant issues during my time working in that lab.
You can then offer to talk with the asker in more detail, if she so wishes, and leave the choice up to the asker. But do not blow off the discussion. Hiding the situation will just allow it to continue unchecked, which is exactly what you do not want to see happen in the long run.
I am very sorry to hear about your situation - that sounds very painful and I wish you the best in healing from it.
Personally, I'd love to just answer truthfully and throw him under the bus since that's what he's done to me several times over these last few months, but I know doing so will hurt me more than it'll hurt him.
Probably the biggest argument for answering truthfully isn't payback (no matter how richly deserved) but protecting your peers from the same experience you had. Against that, as you've identified, there is a risk of retaliation if he hears that you're doing this.
By reporting him you've already taken action to deal with this missing stair. This may discourage him from repeating the harassment and/or improve the outcome for the next student to report, if it comes to that.
I've been in a similar situation (not sexual harassment, but bullying and professional retaliation from an academic superior) and I understand that it's exhausting. Nobody here can tell you whether you ought to do more than you already have done.
With that said...
After effects: we're not speaking to each other, avoid each other as much as possible, I get panic attacks/have some sort of PTSD (Post-Traumatic Stress Disorder) whenever I have run-ins with him, and I've slowly become isolated from the rest of my department because he practically runs it and no student or faculty member wants/dares to step on his toes).
To be honest, it sounds as if this bridge is well and truly burned. You've formally reported him to his professional colleagues. This is a much more threatening step than informally warning off a potential student. At this point I expect the only thing restraining his behaviour is fear of professional consequences if he gets too blatant about it.
To be blunt, whatever you might have stood to lose in your relationship with him by warning off another student, you've already lost it.
What you do need to consider is the risk of giving him ammunition. If you make specific accusations against him, and he or colleagues find out about it, there's a chance that this could be used against you. You are probably best keeping it to things that are incontrovertible fact.
Other answers here have suggested deflecting the question. I'm not a fan of this; some people may notice the deflection and read between the lines, but not everybody catches that kind of subtlety.
You might consider something along the following lines:
"Yes, he was my advisor for a couple of years. I'm afraid we don't have a good relationship and I ended up making a formal complaint against him, so I'd prefer not to discuss the details."
You're not making any accusations there, so it'd be hard for him or anybody else to take this as an attempt at escalation. At the same time, it lets her know to keep her eyes open for trouble. In the event that she doesn't take the warning and he harasses her, it also lets her know that there is already a formal complaint on record against this guy, which may help then.
This answer is focused on keeping the OP well and advancing their career.
I fully support the right of OP to warn others about the professor, but it need not be done every time his name comes up in conversation. Having to warn all askers is a huge and unfair burden, and the topic of OP's grad study is [awesome topic that fits with the conference], not [setting the record straight about Dr. --]. Further, sometimes hearing or talking about a person can trigger PTSD (possibly part of the "deer in the headlights" moment--fight, flight, or freeze), and certainly telling her experience in any detail, in a convincing way is even more liable to bring up those memories.
Even in the comments on this SE question, people asked questions to try to get at the nature of "crossing a line," offering that maybe the OP misunderstood, that it would help to know what kind of line was crossed. As Thomas King commented, "in every instance I have known, people have favored the perpetrator (at least initially)." People are often skeptical and don't want to believe people who have gained their (professional) respect can do awful things.
OP should not have to spend valuable time at a conference rehashing Prof. X's misdeeds, convincing skeptical strangers who started with idle curiosity, and being remembered as "that student bitter about Dr. --" rather than "the grad student who's pursuing [awesome topic]."
A friend leaving an abusive advisor was told by someone in charge of her department that students are admitted on the basis of many opinions, and during the admissions process other professors acknowledge that this is someone they think would add to the department. This may not be universal, but your identity as a student is linked to the department/university, not just your former advisor. Further, your research interests have a (potential) audience and community of scholars much broader than Dr. --.
The main advice for the conference: focus on the positive! For the most part, you are dealing with strangers who are rarely going to interact with Dr. --.
With or without a new advisor, you could say:
I've worked with him, but it seems that our university has a [growing | large | passionate] group of scholars interested in [awesome topic].
If you have a new advisor, you can answer with:
I'm really excited to be working with Dr. Y.
I chose to work with Dr. Y, who [brings expertise on velociraptors | has a high energy kitten lab | is a great collaborator].
What you choose to praise about your new advisor may be an implicit critique of your old advisor. Calibrate what you say for how much you want to throw shade on your former advisor.
For a fellow grad student (e.g. who might apply to work as his postdoc or otherwise work with him in a junior position), it may be appropriate to sound a bit of a warning; for someone who already knows you and you trust, you can similarly also let on some misgivings.
If you're talking to an established professor you do not know, try not to waste time talking about your old advisor and the past, but instead talk about what you're excited about.
- Talking to senior scholars is a key opportunity at conferences!
- Only if you know the person is likely to be sympathetic (e.g., just gave a speech about rooting out bad behavior in the discipline), you might choose to reveal something to them.
- Someone established is more likely to already know and somewhat trust Dr. --, knowing him in a context that had a very different power differential, and you would be much more likely to get disbelief and/or be seen as unprofessional for airing dirty laundry.
If a stranger--someone you don't trust yet--asks about what happened, you can jokingly defer. Or even if there's someone who you trust but you do not feel like going through the torture of talking about it, you can still change the subject. Some possible ways of saying this are:
How much time do you have? Really, though, I'd love to hear more about what you're working on.
I value qualities in an advisor beyond a great research reputation. But I don't want to spend all my time at this conference talking about him, when I really want to talk about that amazing keynote.
Get in touch if you're thinking of applying; I'd be happy to talk more about the research environment in my department. But for now, I wanted to ask you about X...
The way you defer does not have to be joking. To prevent unwanted follow-ups, though, you should try not to have an air of mystery about it. You could try being extremely dull about the answer.
Oh, the story's more about paperwork than about [field of study].
Nic Hartley suggests in the comments another approach, that is candid but limited:
Unprofessional behavior happened and HR is looking into it.
He's right that this approach is "honest, makes it clear that nothing is set in stone ('looking into it', rather than 'he was fired'), and gives some idea as to what happened, without requiring OP to go into possibly painful detail." However, even when asked point-blank, there is no duty to answer personal questions. One could also just say:
I'd rather not go into that.
Comments are suggesting that this answer is artificially happy and papers over the problem. I was wrong to say in the original version that you should fake excitement, but if you can feel excitement about the conference, then hold onto that and don't let the bastards get you down! There are a range of legitimate options, and you will know what feels like the right direction in the situation.
Few other answers really acknowledge the stressful reality of your situation and PTSD. Extrapolating from my own related experience, I imagine that even being at the conference--perhaps hearing citations of Dr. --'s work or remembering how he explained a concept someone's talking about--may feel like a minefield. It is so easy for thoughts to return to a trauma and force you into those same feelings, especially if you are having to talk about anything remotely related. It can also lead to worry that the road ahead on your career is still under Dr. --'s control, leading to panic or despair or anger. And in my case, having had the emotions brought up once made it more likely that panic or tears would return later in the same day. Especially when the cost of disclosing something to a stranger is spending most of the conference holding back sobs and periodically leaving to cry in the bathroom, it is clear that you are not morally obliged to take on that burden.
(In my case, therapy has helped a lot, as has learning how to reclaim my identity in that realm, separate from traumatic incidents. You have plenty of time ahead to share your truth on your terms. In my own case (with different details), I have discreetly warned many people one-on-one and encouraged policy changes that make it unlikely for similar things to happen again.)
Good luck! Especially with the PTSD, I hope you can get some counseling resources to help. The power dynamics and uncertainty that grad students face makes a lot of problems especially stressful, and it can even be common to doubt oneself when lots of people are questioning you. You are not morally obligated to disclose this beyond the formal avenues you are already pursuing, and you have a scholarly identity and future that's not just tied to Dr. --.