I just noticed that when you knock Midir off the bridge, he doesn't fall into a dark, bottomless pit that eventually leads to the underground chasm we fight him in, but he's actually falling into the black swamp. I found this out when I was tooling around the half-submerged building guarded by multiple Ringed Knights, and noticed that the black swamp extends all the way up to the base of the cliffs that the Church of Filianore is seated. In fact, if you look carefully from the bridge itself, you can see the slightest hint of watery texture in the pit.
Doesn't seem important at first, but this also means that we, too, managed to pass this barrier of black water without knowing and somehow ended up in the chasm. Most people associate this black water with the Abyss, which makes perfect sense, but I wonder if it could instead be a representation of the Deep that Aldrich was obsessed with? After all, it says in the trivia here that the water in Midir's arena is filled with the same ruddy body parts as in Aldrich's arena. Even more concerning is the fact that Aldrich's habit of cannibalism lines up well with the Locusts' obsession with the feast (and yes, Gael's consumption of the Dark Soul, but for the sake of argument...).
Last but not least, there are pools of this same black fluid in the Dreg Heap that spawn forth the Murkmen, and if you get a good look at them, they're all covered in barnacles. In the description for the Soul of Aldrich, it says that Aldrich had envisioned the coming of the Age of the Deep Sea, suggesting the Deep does, in fact, have something to do with a body of water. The Deep is also associated with Human Dregs, which the Murkmen drop/cast/transform into, and the Dregs are also said to be the "shackles that bind this world," acting as a sort of foundation for existence.
Or, maybe the black swamp thing was just a total oversight on From's part, and I'm talking***** out of my mouth.