Can current employer create issue with my H1B transfer?

Hello All,
I need expert guidance, thanks everyone in advance for answering my question.

I'm working in the US since past 6 months on H1B and my employer is literally sucking my blood by making me work for several clients at the same time, I work overtime, sometimes in the middle of the night and not paid for it at all. On top of this he treats everyone like animals pressurizing us all the time for stupid tasks, I'm totally frustrated and can't take it anymore as it is now affecting my health and personal life. I would never take this job if knew all of this in advance. I'm now thinking of going back to India and work there but at the same time I feel all my hard work and efforts that I put into this job/interview/visa etc. will be wasted and it will be a big setback for me professionally so I'm considering offers in the US also. When I informed my employer about leaving the job he first started manipulating me and then threatened me indirectly about legal consequences etc. now I have these questions-

1. Can my employer in anyway cancel or create any issues with my visa status?
2. Can he force me to reimburse for money that he spent on visa application, training (which was never provided, in fact I trained so many others) etc?
3. How unpleasant this situation can get? 

Reason why I’m asking these questions is because If this isn't worth it then i would rather save my efforts and go back to India instead of taking more stress. 
Please advise on this.

I am not able to believe your comments about your employer. What does your employer company do?

As far as your question goes, It depends on the contract you signed. He cannot stop you from transferring to other company. But you have to do it in a calm way.

Your employer can cancel you H1 by requesting USCIS. If the cancel request is approved before your transfer request , then you have to leave the country.

Your employer may file a case against you and you may have to pay the compensation if such terms were written in the initial contract.

You can contest the case saying that you are ill-treated(provided you have proofs of your employer behaviour)