Dungeons can look however you feel! As dreams themselves sometimes have no rhyme or reason, you could technically have an entire dungeon that's upside down, or goes from being outside into a small door that leads into a volcano lair.

As it also uses the Dream Collective, you can even base them off canon dungeons!

While you can have a map, you could just technically generally describe what it is, and what forms the shadows would take (this can include monsters from the canon as well). The shadows themselves would just be black forms that cast no light, like Vantablack