The Black Empire was a world-wide spanning Old God civilization, at the height of their power when Azeroth was still young.

While, scenarios were not brought forward into future expansions in favor of increased phasing to allow more dynamic quest elements within the shared open world, it can be considered as a precursor to several other solo and small group activities throughout the years including the Deaths of Chromie, Warfronts, Island Expeditions, Horrific Visions, and even Torghast. Of course, it's all speculation at this point, but the Mists of Pandaria inspiration for Dragonflight makes this idea feel similar to MoP Scenarios - short dungeon like instanced activities with themed gameplay and storytelling elements. Presumably through the result of some bronze dragon shenanigans, this would appear to indicate that we get a chance to go back and experience some of these events, to see how they influenced the rest of our world. The maps themselves are mostly identical to those already seen in-game, but the internal names for their zones provide clues that these aren't quite the same places we remember and some of the maps even include level ranges of 68-70, suggesting late or max level activities in Dragonflight. It's not exactly clear what Grand Adventure Time is right now, and it's likely just a placeholder title anyway, but the time bending nature of these maps suggests a new Bronze Dragonflight or Caverns of Time themed activity.

Mortal empires map