Employee Bond - They have held my 3 months salary and didn’t provide experience letter

I worked as Software Engineer in IT company. I was already an experienced employee before joining the company and received no training. This company had got me signed 2 documents - an offer letter and a verification form. In offer letter, it’s written that obligation to be in the company is 12 months and in the verification form, it’s written I intend to be in the company for at least 18 months. The service notice is mentioned as 3 months.

I resigned from the company after 12 months and served 3 months notice period. But the company is saying my resignation is not accepted and I have to fulfil 18 months there. I told the company that I will not attend the company after the notice period and I did so. Now the company is threatening to sue me. They have held my 3 months salary and didn’t provide an experience letter. What should I do?

These are two documents which I signed.

According to the documents you have given you are required to work for a period of 12 months or pay the training cost of ₹ 1 Lakh. I didn't see the 18 month period in the second document, so don't know where that came in.

Intent to work is not a promise of a compulsion and the period specified in the offer letter stands.
That said, I don't know what you can actually do.
Taking a legal route is very expensive and time consuming. I don't think the labour officer will help as you are probably not under the definition of worker. Further, you will need to refer to them for BVG in future, so coming to an amicable settlement is always better

Thanks for your response. The statement regarding 18 months duration is mentioned in last page of the verification form.

I already tried settling things in company on a positive note but the company's responsive has been threatening to say the least.

I am planning to take legal route, I understand the consequences including time and money, but I will rather fight than letting this company extorting me.

I just want to make sure that I didn't do something wrong.

You need to speak to a lawyer, but from my point of view, such declaration has practically no value or enforceability in law.