Remember YOU MUST go to the brothel first, and be rough with a girl (Dasiy).At the brothel hire Daisy, ask for a blowjob, force her to deepthroat, do this twice, then explore the Town Square, the Mouth Fiend (Name is Lura of Buccara) should come up to you acting as a fangirl, there will be a skill check (Perception 8), fail that and you'll get taken to a strange castle , well a 2 tile area, 1 tile "forest" the other "strange castle" go to the "strange castle". Once there answer no to her first question (options, yes, no, insult her, go with no) the next 3 questions I don't think really matter, next when you get a chance to get free "actually reshackle yourself" the next choice should be if you want it deep or hard, honestly I don't think it matters.
I literally did this while typing this to make sure it works, it does.
It is, there are some parts of the game that are just like that (mostly achievement related), but once you know how to do it, it's a quick thing. This is the only Mouth Fiend I know of, as her scenes show up in the "Pervert" menu option under "Mouth Fiend" assuming you haven't already done it, do it on Ironman mode for the extra bonus point.