So, a couple years back I had a terrible manager. He was a nepotism hire, and didn't even have a degree, let alone one related to the technical department he managed.
I unwittingly made management look like a joke with my projects and studies, and he took it personally. He made my life a living hell, and set me up to fail, and fired me.
Now, years later, employers are calling him up and he's badmouthing me so I got ghosted for a second round interview!
Fuck this stupid fucking chud of a man. He made my life so bad I considered checking out of life, left me unemployed for half a year, and I got saddled with 10s of thousands of dollars of debt from said unemployment.
Should I warn employers of this bad egg? "Hey if you contact this employer, my direct manager had personal grievances with me"
Or do I just say nothing since that warning would cause hiring managers to skip over me?
If you can get a statement from one of the people that made the call that is the only way I can think of to get proof.
Unfortunately libel is extremely hard to stick in the US but it is absolutely illegal for someone to do what your manager did.
Sadly I have been in the exact situation you are in because I got fired from my last job in the midst of trying to form a union.