Legal Action against leaving company without relieving letter

Hi I have officially resigned from my ex-employer by giving 30 days serving notice out of 90 day notice period but company did not accepted my request and keep stating that as per the "Service Rules of the Company, you are required to give 3 months’ complete notice and to serve the company during the entire notice period .In view of the above rule, you cannot be relieved before expiry of 3 months of clear notice of your resignation" Accordingly I countered by stating that as per agreement held between me and employer during my joining it is clearly stated that "either party can terminate the services by giving 3 month notice or 3 month basic salary in lieu thereof to the other party". But company did not respond thereafter and I have left company on last date of 30 days by giving email to all stakeholder i.e. CHRO, BH, HO, Manager, HR team that its my last working day and submit the handover and company device to manager but till date no one responded or tried to contact me. Now my current new employer is asking me for the relieving letter or at least confirmation on last working date so that it does not come under dual employment. What should i do and how to tackle this condition. Ex-employer has threatened me to make it an absconding case during my serving period there.