Wants to leave after joining 2 days but employer Black mailed

I joined in a health care organisations where they had given me a offer later regarding to employment contract and details.and also they holded my original certificates in the name of verification and I also accepted. But after joining in 2nd day I thought I am not good fit for these role and also I thought myself i am forcing to work. and in my family things also not in a good way I asked my employer explanation of all the things, They said if you leave we will destroy ur certificates and u have to face legal actions. I clearly said i will serve untill u will get my replacement, But, they are saying mistake was in ur side u have to take replacement then u may leave. It's been 3 weeks they are not taking any newones and even I refering they are not recruiting. I am mentally stressed and developed some anxiety related issues, don't know what to take next. And they are saying if you leave then u have to pay 3 months salary, Still I didn't get any single rupee from their side Can suggest what should I have to take next?