Builders did a fraud in giving possession & now he is dead.

My case is unusual than others, I have Purchased a land from the builders who was going to give me a full constructed House Possession. With me, my 2 Friends invested there money too in same thing. At that time he showed us a ground on which he was constructing but as time passes he did a fraud and While getting the map Verified he Never mentioned which house is next to which. I.e he sold the same Land to multiple people and made more registry than the actual land. Now after some years he was not able to give us possession so he kept pushing to settle for different property and expensive property if we fulfill the amount difference. Fast-forwarding, The builder passed away some time back and now his sons are managing his Company, and after Talking with them they agreed to settle this soon but again they are just pushing dates. And now it's been a year with son's too. I don't understand at what section should I file the case. I'm really Confused and it's Sad that how a person can do this kind of fraud and not take any accountability. According to me, if i person is going to take Father's Property and profits than he should also be liable to pay for his debts. Please Answer and help in any way you can. Thanx.