LAND CLAIM BY THIRD PARTY

You have to do two things:-

1. Ask your Land's previous owner that he has mortgaged this land to the third party or not? 

2. Ask the third party who is claiming the land for the original mortgage paper. 

If the owner will say no then ask to the third party and if third party will not be ready to show the paper to you. Then, don't worry and make a formal police complaint. 

If the third party show you the original mortgage paper then file a suit as well as FIR against your Land's previous Owner.