When the mortgagor of such an undischarged mortgage and those having his estate in the premises have been in uninterrupted possession of such real estate for 20 years from the date thereof, and it shall appear that such mortgage was not given to secure the payment of a sum of money or a debt, but to secure the mortgagee against some contingent liability assumed or undertaken by him, and that such conditional liability has ceased to exist and that the interests of no person will be prejudiced by the discharge of such mortgage, the mortgagor or those having his estate in the premises, or any of the persons to whom a similar remedy is granted in section 6104, may apply to the Superior Court in the county where the whole or any part of the mortgaged premises is situated, by complaint setting forth the facts and asking for a decree as hereinafter provided. If after notice to all persons interested as provided in section 6107, and upon hearing it shall appear that the liability on account of which such mortgage was given has ceased to exist and that such mortgage ought to be discharged, the Superior Court may enter a decree setting forth the facts proved and its findings in relation thereto, which decree shall within 30 days be recorded in the registry of deeds where the mortgage is recorded. Thereafter no action shall be brought to enforce a title under said mortgage.