Threatening that they blacklist

I joined in a company in 2021 june. While joining there is no any written bond or documents to be signed but they just said u have work for 21 months and then u will get a relieving letter.. If u go in between 21 months u won't get relive letter and u can't join again in the company. They said if u go in between it will be considered as abscond that's it they said in jan 2022 I got a other company offer so. I decided to quit when I said this to my manager and hr they are insisting me to work for 21 months if I fail to do they saying that they will take legal action and they report in NSR and blacklist me so that I won't join anymore company in future .. So they are threatening me... But they didn't say like this while I'm. Joining they just said if u leave in between u won't get relieving letter this is what they said... But now they are threatening me.. But I didn't submitted any of my original to the company becoz as it is covid I just upload soft copies of documents and I not signed any document in person... So I thought it won't be a problm.. But they threatening me what should I do now