Similar to a UnmentionableIncident .
Phrases often used by Joseph Conrad to imbue a moment or event with profundity specifically in Heart of Darkness and Conrad's use of phrases such as 'brooding over an inscrutable purpose' or 'unspeakable rites'. English was Conrad's third language.
Kurtz: "Do you find my methods unsound?"
Marlow/Willard: "I don't see any method at all"
Kurtz: "The horror, the horror".
See The Other.