Who should be the rightful Owner ?

I bought a plot of land with a small house, Sale Deed is drafted by a lawyer with all necessary clause and the same I made land ownership certificate and Land Certificate from Colony Chairman, now third party is claiming over my land with a written mortgage agreement from the previous owner and was dated 4 months before I bought the plot, but the previous owner who sold the land to me did not tell me anything about the mortgage, My question is who has the title over the property, the mortgagee /borrower or me?