I don't know what Obsidian has in mind for the Endless Paths, but based on the fanciful illustration, I'm imagining a huge dungeon where everything is rock hard and the floors have been designed by a human being and there's cool stuff at the bottom. Stuff like the giant statue spanning multiple floors, arcane architecture, and crystalline growths in the depths sort of allude to this. I doubt it's an accurate depiction, but if it indicates a design direction for the dungeon, I think it will be a standout feature of the game.
I also assume it will be an optional (post-game?) dungeon and definitely not part of the main story. Or at least, you won't be required to finish it in the main run, although you may be taken nearby or even to the first couple of floors. Usually dungeons of this magnitude aren't wasted on those who abandon the quest, but Obsidian may defy my expectations.