title_body: Grammatical cases occurring only in conlangs
upvoted_answer: I think the best conlang to look for such cases is Ithkuil. http://ithkuil.net/04_case.html contains a list of all cases. My [short] research suggests that some of them (e.g. navigative for "noun relative to whose vector, arc, or trajectory of motion an event takes place" or allapsive for "amount of time that expected to pass between the contextual present and the time of a future event") aren't attested in any other well-known language. I may be wrong, but it seems plausible regarding Ithkuil's goal of expressing extensive detail.