Notice from employer

Dear Sir, I joined company on [deleted] I haven't given any oppointment letter and I didn't sign on any agreement of employment. I was put in project in other state. I have resigned company on [deleted] by stating my last date will be on 20-09-20 in official mail to the company. Later they asked for under signed Resignation letter.The same was done scan copy sent in official mail by trailing old mail. Anyway they didn't acknowledge the same.. As reliever didn't reached I waited on [deleted] they sent the reliever .Afer two days I got request mail from CEO to continue till the project closure (also he mention any expectation can be renegotiated) So I have sent some of my requirements to My reporting manager . They agreed alll point and sent the same point to in mail asking me to undersigned and sent back . I have signed the same and sent it through official mail. But all were only my requirements( only points were mention nothing g written about notice period or employment agreement)in that 2 points were failed by them I decided to quite. I have sent mail that to releave me and after 3 days of mail i handover all things to My reliver and took sign from him and left the site by sending all in mail to the hr and MD on 03-10-20.Even I have formally informed client of my releiver in mail . So now I got notice from employer claiming that I haven't served notice after new terms agreed and I haven't gone to office for reliving procedure note done and they mentioned that they will be sending legal notice for negligence of work and putting company financial loss and reputation.. What to do now plz advice