Received a legal notice from my previous employer

My previous employer has sent (emailed) a legal notice to me, telling me to pay my outstanding dues. Background: I left my previous company because I was diagnosed with severe back pain and weakness in my body. I requested for a notice period waiver before I left the company, provided all the necessary medical documents, and I was relieved. After a while, I received my full and full settlement email, in which they included the amount against my unserved notice period. I emailed my manager about this and he told me that my notice period waiver request was rejected. This was the first time I ever heard from the company that they rejected my request, and I didn’t even receive any communication from their side ever since regarding the status of my request. Had I been informed; I would’ve taken a different action. HR told me that I can still ask for my skip level manager to approve this request, and the amount will be waived. I emailed to my skip level manger as I was instructed, but despite emailing him multiple times, I didn’t receive any reply. Also, I’ve returned the company’s assets (laptop, bag etc.) already, but still in the legal notice, they’ve included the amount for the company’s assets as well. They've been harassing me by sending emails and letters to my address ever since I left the company. So, they’ve included the amount for the company’s assets (already returned) and my unserved notice period, both of which shouldn’t even be there. If the amount listed for the company’s assets is removed then the total outstanding amount is around 1 lac. What should I do now? how do I proceed?