Should I take threats from a former professor seriously?

This sounds terrible, and like you said, I definitely think asking her to write a letter of rec was a mistake. However, regardless of whether she tried to sabotage you or not, you've been admitted! Congratulations.

Now is the time to sever all contact with her and move on.

You left a record with your previous university about your issues with her, and if she was going to create problems at your new school I really think it would have come before you were admitted, especially since she was a letter writer. Whatever she did or did not say, they gave you a big vote of confidence in offering you a place. Time to focus on the next stage.

Good luck!

First of all, if these actions can be (or already are) documented, start a case on that professor. This is mobbing, and breaking a few global ethical rules.

Asking her for a reference letter was a mistake, but it is not un-recoverable.

Personally, I do not believe she would have major effects on your status of acceptance. Academics get to know a lot of people and they are aware of these kind of people in academia. Hence, they will not judge you by only one letter of reference.

If, on the other hand, an institution judges you by a single negative letter of reference, then I (again personally) would doubt the credibility of the programme. There are many ways to get to know a potential PhD student, such as reading your work, Skype interview, face-to-face interview, talking to your references on the phone etc. Taking a decision regarding to a single reference letter is not very professional. Another possibility is that the institute might have already decided not to hire you, and they might use the letter as an excuse to dismiss your application.

It is true that there might be some cases where an institution can interfere with your reputation, but those cases are either documented, or reported unanimously.

As a result, if I were you, I would

• Press charges both legally and academically.
• Ignore the person during my application period,
• Avoid mentioning her to my potential supervisors unless I am asked.

I had a similar case where a project supervisor claimed that he "hired me despite all the negative feedback from my superiors." Ended up getting nothing but a decrease in his reputation.

If this person is as bad as you say this will be well known, and she is probably less influential than she thinks.

My advice is from now, just ignore her. Don't read or reply to a single email from her. Set an email filter to delete her emails before you see them if it really bothers you and get on with your life. She'll soon be a distant memory.

Alternatively, why not deliberately foster a strong enmity, having an enemy you can righteously work against can be a great motivator in life, and provide much entertainment, provided you like this kind of thing.