Settle dues of due to non-serving of requisite notice period.

Dear Sir | Ma'am I was joined in aakash educational services pvt ltd as accountant in March2017 without any agreement. later on1st May 2018 resigned my job & joined in another company on 2nd may 2018. 20th September 18 i asked Relieving letter through mail, after my mail they are giving notice Already i was mentioned in resign letter my last working day . They aren't responding till the date. After 5 months aakash company giving notice for settle dues due to non-serving of requisite notice period. They are demanding amount , otherwise taking an legal action against to me. Is it compulsory for payment ??. If am not paying that amount what happened?? Please suggest me on this.. TIA Regards Venkat