Asked to write a letter of recommendation for promotion for an abusive former professor

Get your former advisor to sign the letter now.

After that has succeeded or failed, you should contact the person who runs this process and ask them verbally how the letter will be used and what it contains. Probably this is a dean. Probably the dean already knows about your advisor's behavior. They might be seeking a letter that will help them get rid of your former advisor. At well-run universities, these letters are not requested from random students. They come from known students.

You should write an honest letter.

There is an option you do not mention: You can "damn with faint praise." This is done by writing a letter which says nice things about a person, but none of the nice things are relevant to the person's job. E.g. They are funny and well-dressed. I don't recommend this but it is available to you. If done carefully, it can prevent promotion while giving the illusion that you intended to help the advisor.

I read this with interest bc I had the same experience and felt the same way with one of my advisers who also happened to teach several of the classes for the program I was in. I was going to be a teacher and he made it so miserable that, while I finished the program, I decided not to pursue teaching. I had one last requirement, to create a personal portfolio to have when I interviewed for jobs, and he wanted to abuse me one more time so kept making me jump through ridiculous hoops. I finally refused any more abuse, so he refused to officially sign off that I completed the program and the University wouldn't help me bc he was tenured and untouchable. (They agreed with my concerns!) It's 10 years later and I still have this unresolved thing I have to explain when pursuing new employment.

If you contribute to his/her tenure, it will only embolden them to mistreat students even more and it could be life-changing for many of them. I suggest you follow some of the great advice above that leads to a path that you don't support the tenure while still ensuring your needs are taken care of.

Postscript: A few years after I moved on with my life, I heard my adviser died. Maybe you'll get lucky...sounds cold but when people are this despicable, they don't deserve kindness even after they leave this world.

It seems that your core issue is that you've got three different things tangled up into one mess:

• You want a letter from your former advisor to support your immigration process.
• Your former advisor was abusive.
• Someone is asking you to write a letter supporting your former advisor's promotion.

Personally, I don't think any of the three options you presented in your question are complete in terms of addressing each of these three things appropriately. You may be better served to separate the three and conquer them individually.

• Determine if you have other sources for a recommendation, besides your advisor. Perhaps the letter could come from a department head or another professor.
• Decline the request to write a letter recommending your former advisor. Whomever is asking will likely get the message. You certainly don't want to actually write a positive letter, and a letter of recommendation is not the ideal channel for dealing with abuse. Further, it would be legitimate to indicate that you don't feel comfortable writing a letter of recommendation, since you are still (indirectly) dependent on this professor writing a letter recommending you, and hence there is a conflict of interest. Which leads to,
• Follow up with the dean of academic affairs, department head, or other appropriate authority within your university in terms of reporting the abuse you suffered.