Relieving and Notice period

Respected Sir, I worked in an MNC for 6.5 years and been suffering from cervical spondylitis from last 3 years, Now the stage has come where I cannot even get up and sit more than 30 minutes before the system and work. I have explained the situation to the management and requested for LOP and they have denied as my leaves are exhausted. Company has put me under immense pressure and lot of mental torture during the tenure. I have resigned in the best interest of my family and myself. I sent my resignation letter through email and requested the management to relieve immediately.However, they are not ready to relieve me and are forcing me to serve the notice period. In the period, they have sent the absconding letter that I have not informed the valid reasons for the absence. I have explained them through calls and emails and urged, but no luck. During the last 3 months, my manager has put me under severe mental pressure and they have threatened me a lot if notice period is not served. Kindly suggest my course of action if I have to go legally. Thanks in advance for your help. Regards, Mahender