procedure to challenge sale deed in a court.

Dear sir, how to challenge the sale deed executed in sub registrar office by cancelled grantee to third party in 2005.the alleged registered land grant canceled by divisional commissioner in 1979 but we didn't knew about it.we are cultivates of this land since 1975.RTC stood in my grand father name till 1998.but after 1998 thahsildar made RTC in favour of canceled grantee without consider the divisional commissioner order passed in 1979.now where should we file case for cancel of sale deed