First serve them a legal notice and get the land measured through the local land office. If not then arrange surveyor who can tell the land demarcation which you have purchased. In any case if the land is with you and you want to retain it then the best course of action will be to file a suit for declaration and the rest will be taken care by your lawyer.
File a suit for declaration in court. For any further assistance, feel free to contact me on my e-mail "email@example.com"
Shubham Pabbi & Associates
To answer this question I would like to see. Copy of sale deed
You can take my contact number through lawyered and can contact me