Employer holding FnF and Relieving Letter

I gave my employer a verbal notice of resignation on 6th October. He kept pushing the discussion and asked for more time and eventually I gave a written notice in 15th October. 14th November(sunday) was my last day of notice period and there 12th November (Friday) my last working day. On 11th November my employer comes to me and promises a 100% hike in salary and begs me to stay. I accept is verbal promise, immediately after which he replied to my written resignation as void due to acceptance of the hike to which I reply thank you for the offer, looking forward to working with you. I finally get my new contract on 9th December after a month of delay and it claims that I have a non terminate clause which says incase I leave before 12month I have to pay the difference in salary back to the company, including bonus and serve a 3 month notice period, along with many other clauses that weren't mentioned the me before. I rejected the offer and said my old resignation still stands but my employer did not agree and said I need to do 30 days notice period again. I did a professional courtesy of doing handover for the next one week which was agreed upon by both parties and now my employer is holding my fnf and relieving letter as he expects Me to serve additional 3 weeks despite doing the handover as I refused to sign a new bizzared NDA he came up with which claims I cannot work for 6 months after leaving the company and many other such clauses that do not match the LOA. He is now continuing to threaten me that disciplinary action will be taken against me. Can my employer actually take legal action? Can I take legal action for my fnf and relieving letter and mental harassment caused during the process?