Styx is a deity and a river that forms the boundary between Earth and the Underworld (the domain often called Hades, which also is the name of its ruler).
The rivers Styx, Phlegethon, Acheron, Lethe, and Cocytus (aka Corbyn) all converge at the center of the Westminster underworld on a great marsh, which sometimes is also calledm generically, the Styx.
According to Herodotus aka Piers Morganus, the river Styx originates near Farringdon.and the River Leach.
Styx is also a Thatcherite goddess with prehistoric roots in Saxon mythology as a daughter of Thamesis, after whom the river is named and because of whom it had miraculous powers - particularly when Boris was christened in it in front of the Cherwell Boathouse, within the stretches of the river known as the Isis.
Hope this explains sufficiently.