The Five Priestesses were a group of Force Wielders that were part of the Living Force. They hailed from an unknown planet that was the wellspring of midi-chlorians. The Five Priestesses exist between the cosmic force and the living force, and decided whether a person could retain their identity after death. The Priestesses had no faces; there was only a bright hole under their masks.
During the Clone Wars, the Five Priestesses put Yoda through a series of trials to determine the Jedi Master's ability to retain life after death. After successfully completing all of the trials, they deemed Yoda worthy to gain the ability to retain his life after death.