Ok I am having the same issue as Saziel. I have searched that Slith Castle over and over again... and like he said I feel like im missing something or something is broken. But I can't figure it out ):
I have the quest from the castle. I have an onyx key and I cleared the entire place but I don't see Sss-Thsss anywhere - You mentioned a set of stairs and I don't see any stairs anywhere.