Serving notice period

I used to work for a finance company as state head and used to handle sales team where loan approval and process was not under my control. I was not happy with the increasing work pressure and long hours working around 12 hours in normal day and 15-18 hours in last week of month. I got a good opportunity and resigned. My organisation had a 3 months notice period but the new employer had asked to join in 1 month so I requested them to relive me in one month but they didn't agree and tried to retain me. Finally I left the organisation after one month, handing over all the company assets. I also sent them a mail stating that they can deduct my notice pay from the 45 days salary they have with them and 30 day earned leaves I had in system and I was ready to provide further amount if required but they didn't agree and threaten me that they will ruin my career. As my new company doesn't require relieving letter I joined there and now working fine. As I wanted to be clear on my experience, I sent a mail to HR demanding my relieving letter but they have sent me a show cause notice signed by HR head. As per them they have got financial loss due to my early leaving and they will take legal action. I have reverted to them that I was not directly sourcing any business nor I had any power to approve then how can I give financial loss. If some thing has gone wrong then they should take action against the sourcing dealer or the credit manager who has approved as I was directly not involved in sourcing and I had 2 layers of reporties who use to interact in market. Please suggest what should I do further and can they really take any legal action against me.