The primary antagonist in
The Masque of Mandragora is not only a unique entity, but one that’s more of an abstract threat than something we could stat up; it produces visible effects and controls people, but it doesn’t really qualify as a “monster”. Following that story, though, is
The Hand of Fear and Eldrad. Now, Eldrad himself is also unique, in that he’s the last surviving member of his species, the Kastrians, but because we can throw time travel into the mix, that doesn’t mean we could never meet another. Having said that, the Kastrians do not return, or even get much of a mention, elsewhere in the TV series. Indeed, their only other appearance to date is in the audio story
Eldrad Must Die! which is, of course, a direct sequel.
