Employer is not giving releving letter need legal advice

I worked in Company 'X' for 1 year 10 months. I got new job with better salary so I sent resignation letter to my HR. After they did not acknowledge my resignation mail. I sent one more reminder mail after 4 days, they said "we accepted your redignation but you will get salary after serving notice period. But they did not confirm my resignation date". After that they gave me lot of work pressure. After serving 42 days notice period(usualy 2 months notice), I told my HR to confirm my resignation date. On that time they told to me to complete project. It takes 2 months to complete project. I did not accept it. They argued with me and terminated me for that silly reason. They accepted to give salary and payslip. But they did not give resignation and experience letter. Still they did not accept it. New company asking this documents to me. What will I do? Shall I take legal action against my past company? If I take legal action they may mention bad feedbacks in my releiving letter.. I needed legal advice. Please give me legal advice.