The Dead Jailor was once the master of the Palace Dungeon, and his Skeleton still lies therein. Scarlett will need to speak to him about getting the key to the jail cells. He will help her in exchange for a favor, the Lost Love quest.
Because he is, well, dead, to speak with him, Scarlett will have to invoke The Passage mental skill to see his ghost (passive Eye of the Lost skill). That, in turn, will allow her to invoke the passive mental skill The Whisper to speak with him.
The Dead Jailor can be found in the Palace Dungeon, at the upper level in the large, round, central room. (Click on the map to enlarge it.)