Employer not relieving from the job

Have been working with X company since 1 year 3 months. They wanted to me to shift to a different function to which I was not comfortable. During this time I got a new job offer which suits me well. Since the new organization wants me to join soon I put a 15 days notice period to my employer and asked to adjust the remaining 75 days of my notice period with my salary, remaining leaves and incentives for the year. Have completed the exit interview and handover process but X company is not giving me my relieving letter as they think that I have not given a proper handover and much of my work is pending. I have stopped going to office and will be joining the new office soon. X company has sent me a mail and asked to resume work or they will take action. What action can they take? Is the appointment letter on the company letterhead with my sign legally binding? Should I send them a notice of mental harassment before they send me one?