Remedy for Act of fraudulence by landlord

I purchased a land from Mr.X which got registered at Howrah DSR on date 22.11.17. He got this land from his father (inheritance) via a SETTLEMENT in 1992. I did carry out a searching for last 20 years (from 2016). Nothing came out. Seller has this land registered in HIS name in BLLRO LAND RECORD MUTATION, he paid the tax as well. Now his brothers revealed that their father made a separate REVOCATION DEED followed by another SETTLEMENT in 1993. The land he sold, belonged to their mother. The seller has admitted that he concealed this fact. Now the seller's brothers are BLACKMAILING me to pay Rs.4 lakhs in cash, otherwise they'll resist the MUTATION PROCESS. I've PORCHA of the land in Seller's Name (applied for mutation, under processing), Tax Receipts, NOC (NOTARY AFFIDAVIT) duly signed by Seller and his brothers that clearly depicts they have no issues or obligations on this land against their brother (Seller).