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Van Richten's Guide to Fiends details those few demons and devils trapped in Ravenloft. It talks of their past and also their present, except for one: the cornugon known the Black Duke. Although it talks of his time as a bandit leader many years ago, it has nothing on him in the present, leaving me to believe his physical form was destroyed and he remains trapped in his phylactery whip. Did any other Ravenloft products ever talk of the Black Duke?

As best as I know, for official publications there's just his write-up in VRGtF (re-published in Monster Hunter's Compendium III) and a brief mention in Gaz V (in the section on Ehrendton). On the unofficial side, he's in Children of the Night: Demons.