Everyone likes a good ghost story, so Byron decides to indulge us in the final cantos of Don Juan by introducing a ghost of his own that he calls "The Black Friar." As legend goes, this old monk was ordered to leave his monastery and refused. One thing led to another and the dude ended up dead, but he returned to patrol the monastery many years after his death. Lord Henry thinks that the Black Friar is jolly good fun, as he tells Juan at breakfast:
But beware! beware! of the Black Friar, He still retains his sway, For he is yet the Church's heir, Whoever may be the lay. Amundeville is Lord by day, But the monk is Lord by night; Nor wine nor wassail could raise a vassal To question that Friar's right. (16.40)
In other words, the Black Friar brings a little excitement to a group of English aristocrats who are—let's admit it—pretty boring people. Little do we know that the Black Friar is even more exciting than we imagined, because we find out later that the B.F. is actually the Duchess Fitz-Fulke in disguise. She's wearing a monk's hood and roaming the house because she wants to find Don Juan's room and have sex with him. It turns out that the monk (a symbol of male discipline) is actually a woman who's lusting for a young boy. That kind of reversal of expectations is classic Byron.