It's like wandering through a city shrouded in thick fog, in clinging mist. The shapes seem familiar, you almost recognize them for what they are, but when the mist clears briefly, you glimpse that it was not what you thought it was and see just enough to know that it was something else. These brief glimpses are like a puzzle you forget that you are working on. With each new piece that falls into place, the picture becomes clearer, but you still don't know what that picture is. You were given the pieces without having the box to use as a guide.
Sentient zombies, the first thing that came to mind when I had some real idea of what dwells in the darkness of the Underground. It was shortly followed by a thought of it being more like a variation of the Black Pearl's curse from the Pirates of the Caribbean, but sentient zombies is what still comes to mind when I think about it.
This would make a truly fearsome horror movie, and although it doesn't have a traditional happy ending, the way it does end is fitting.