Not serving notice period.

I have started working on sept 2018 with a company named Onestore India Private limited. The management rebranded the name from Onestore India to Digitub technologies and relieved me from Onestore India on aug 2020 and issued a new offer letter under the name of Digitub technologies. But due to the disputes between the directors the company and winded up and relieved all employees from the role with immediate effect on 10 dec 2020. I came in contact with one of the directors who launched a new company Cyberro technologies under his wife and brother in law on 1st jan 2022 and they didn't provide any single document offer letter /agreement or anything. I was on a payrole of cyberro technologies effective from 1st jan 2021 to Dec 2022. But suddenly got the last 2 months salary from Onestore India Private limited bank account. I decided to resign from the position with immediate effect as I haven't signed any document and there is no transparancy in the policies. After resignation I got a legal notice stating to pay 500000 with one month salary 31k.