HyperRogue certainly fits in the "unique" category. This roguelike has a very special premise: it is set in a non-euclidian world; specifically it takes place on a hyperbolic plane. But if that sounds like techno-babble to you, just think of it as a game set in a world where the normal notions 2 or 3 dimensions work a bit differently and everything is very trippy. HyperRogue is not only a mathematical experiment, it's also a full roguelike consisting of 44 worlds with various gameplay mechanics and an adaptive difficulty and scoring system.
It also looks very cool!
It takes a while to get your head around HyperRogue, but it is surprisingly simple to play.