Like an encounter just like any other, only that it would simply be meant to be unbeatable. And never be a part of progression, maybe a FoS but nothing to profit from in the mean of progression.
I would love for this game to have something completely ridiculous and over the top encounter with extremely unforgiving and painful mechanics, multitudes of phases and it could even be a multiple hours long encounter.
The closest example I can think of is Yiazmat from FFXII, not that difficult but even with the best gear and tactic it was easily 2 hours long.