We’re not quite sure what Duodecim Prologus is, but Square Enix filed a trademark for it in Europe, the USA, and now Japan. That’s a fairly good sign that Duodecim Prologus is some kind of project the house of Final Fantasy quietly working on.
Since the title has the esoteric Duodecim term in it we’re guessing it’s connected to Dissidia 012[duodecim]: Final Fantasy. But how? Perhaps, Square Enix will reveal it during their January press conference.