Stygia, the Fifth Hell.

The complete opposite of Phlegethos, Stygia was either a bottomless ocean covered by an ice sheet up to 3 miles (4.8 kilometers) thick, or a frozen sea salted with huge icebergs buried in a cavern several miles below Dis and hundreds of miles away from fiery Phlegethos depending on which cosmological model was in vogue at the time. According to the Great Wheel and World Tree models, the river Styx cut across the ice forming a channel.
The older model also suggested the Styx supported small but hardy plants and mosses which, after millennia of decay of this vegetation, resulted in swampy areas along the banks of the river. A few floating islands were the only non-frozen ground in Stygia, their peaks wreathed in lightning arcing from the coal-black sky. Where lightning struck, a strange phenomenon called "cold fire" erupted: white flames of extreme cold that "burned" for a short time and then disappeared without a trace.
The great city of Tantlin was built upon one of these islands, in the curve of the swampy Styx, or perhaps on a giant ice floe. Due to the proximity of the Styx, Tantlin was a cross-planar trading post for those brave enough to attempt navigating the treacherous river.

Location under

Outer Plane - Nine Hells