Dark nights in Dubai
So what happens when a tide of souls suddenly can’t get to their destination because there suddenly is a big gap of nothing in the way? They go somewhere else. The creation of the Underworld is as simple as that. Stygia is unreachable, so the caverns below the world where a good second option. Sure, it has the occasional Neverborn here or there, and the Dead Gods that populate it can be a pain in the ass, but it was better than any other options. Twilight is only for those unhappy few who can’t move on and Hell is of course the worst possible place.
In Stygia, a soul is in its proper place. In the Underworld, it’s trapped. It’s still tethered to the Fallen World, but after a while even that connection frays. Ghosts turn from the Depths and drop to the Reaches. After a while, even the Reaches don’t do it for them anymore. They just wither too much. Even the Mysteries are too real and again the only direction left is down. They become shades in the Depths, where the few lucky ones become servants of the Helminth, the unlucky ones are eaten and the really unlucky ones become true denizens of that realm, ever-hungry shades.
Somehow, the Underworld welcomes Kindred and repulses Werewolves. Vampires do not struggle with the Beast during their time in the Underworld and need not spend Vitae to awaken (because there is no sun or day down below). They can even learn to feed on the ghosts there. Werewolves on the other hand steadily lose Harmony and risk becoming (even more) unhinged from spending time in the Underworld.