I recently took a project for which I was supposed to be paid from the vendor they made ne sign upon a blank document and the terms were discussed verbally I paid them deposit out of fear of losing that I was signed upon it as they said it was non refundable so I did this later on they added clauses and terms which weren't discussed prior neither agreed upon also they mentally harrassed me through various mails threatening me many times that they will terminate my contract if I don't be respectful towards them and I have eye witnesses call recordings and also mails were they terms were discuss is their anyway I could challenge them legally in court
In my view, you can definitely file a case of harassment and fraud against them.
You should file criminal complaint against them of forgery, harrassment, fraud and civil case of specific performance of contract (that was verbally made) with the help of those witness,phone and email records.
I think you should file fir or general diary in local PS stating that you have signed on blank sheet out of undue influence or coersion ..so they will be restrained to misuse that in future. And whenever they do that you can use your fir or general diary to prove your point.