1. The house has already been registered in your name & thus you became its legal owner,
2. Make sure the registry has been done properly and there has not been any hanky panky,
3. Now, you right letter to the nearest post office putting the date of your said registration stating in the letter that you have purchased the said house and are staying in the said house with your family from that day,
4. Get the receipt of the letter acknowledged with same date i.e. the date of your registration. This you can do by talking to the peon of the said post office,
5. After that enter in to your house with your family and goods and start staying there,
6. After that lodge a police complaint of trespass against him under copy to the SP/DC,
7. If police does not act, file a petition u/s156(3) of Cr.P.C. praying for a direction upon the police to investigate and act based on your complaint,
8. Eviction suit is a civil suit which will take decade,
9. So, file the above trespass complaint which will be a criminal suit and will be decided much faster.