The "Abbess" is a character in the game Warlock of Lust. She is the head of the Convent of Light where not-so-little anymore princess Alexandra had been placed since she was a child, and almost been forgotten. Which you are going to change of course.
However, while you are at it you find out that the Abbess, whose name is mysteriously escaping us, also secretly is a leader of a very ancient Cult of the All-Mother.
She will admit to this curious fact after casually discussing theology with her a few times (hone your Scholar skill), and then ask you for your support. This will start the questline "All-Mother".