Stygia
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.
https://forgottenrealms.fandom.com/wiki/Nine_Hells
With Levistus in control of Stygia, the harsh, chilling realm became even more unbearable, with a sense of starvation and desperation permeating the denizens and landscape. Princess Levistus's tomb aimlessly floated throughout the freezing ocean, occasionally getting stuck between larger icebergs at the outer reaches of the plane for years at a time and sometimes passing in and out of the River Styx. Levistus was incapable of controlling the direction in which the iceberg moved despite her regular attempts to do so, such as by having sails and rudders attached, and it was said that when the currents caused it to drift in an undesirable path, the light laughter of Asmodeus could be heard. Erinyes kept the tomb safe from above while ice devils on ice floe rafts protected it from below, although the gelugons were known to take bribes from visitors wishing to see the Princess of Stygia.
The enchanted ice encasing Levistus was incredibly powerful and ancient even by the standards of the baatezu. Even if the snow and sleet of Stygia didn't freeze over most attempts to break through it twice as fast as the devils mined, its resistance to magic could best any attempts to summon and teleport Levistus out, with even wish and miracle magic failing. Only persistent destruction of the ice or melting it with fire, magical or otherwise, could permanently alter its shape, but even then it almost immediately regenerated any attempts to free Levistus specifically. There was a magical method by which souls sacrificed to Levistus would empower her and melt the ice and, but only good-aligned beings of fire would cause any significant change. Only Asmodeus seemed to be capable of freeing Levistus from her tomb, and only did so once a year so that she could sulkily attend the meetings of the archdevils in Nessus.
Despite this, other areas of the iceberg were able to be hollowed out, and had been converted into a labyrinthine array of corridors, chambers and vaults filled with ice devil militia and wary guard posts. The tunnels wound upwards and the prison became more akin to a fearsome palace of ice the higher one rose, not that most intruders would be allowed to see it before being caught. The various properties of the ice allowed the guardians to more easily spy on trespassers, seeing perfectly for hundreds of feet in some clearer areas while using the hairline cracks in certain fissures to listen to distorted sounds from afar. Perhaps the biggest threat of the tomb was its sheer cold, preserving corpses of intruders perfectly and using wicked magic to keep mortals alive. Often times, imprisonment in such areas, known as Halls of Frozen Memory, was a punishment for heroic trespassers and typically their would-be rescuers, as well as disloyal or traitorous devil servants.
Those trapped were as sentient as Levistus but had likely gone mad, filling the area with insane, inescapable screams and nonsense, with the rare intelligent soul being unable to converse with others for too long lest the stillness of the new soul cause them to be entrapped as well. Souls were delivered through docks anchored to the outside of the tomb and suffered a similar fate once pressed inside through monitored ramps, forced to keep moving throughout the halls or else become frozen, although many were already mad. Devils throughout the Nine Hells occasionally schemed to steal from the vaults of Levistus, an idea the Frozen Princess encouraged since it netted her more souls, although none had ever succeeded in stealing from her vaults and devils trapped within had to escape or else be hunted by non-devil teams and sentenced to a fate worse than death.
Guests deemed worthy to be invited inside the palace's suites were met with masterfully crafted decorations and furniture of crystalline ice. Accommodations were both piercingly cold in order to keep the archdevil's presence in the minds of those present and fanciful to remind visitors of her class and riches. Offering flesh and warmth to the chilled visitors, Levistus's succubi servants reported any important information they discovered, assuming visitors were being allowed to talk to her and not an intermediary. After scaling a spiral staircase, Levistus could be found 100 ft (30 m) within the peak of the iceberg inside a towering rampart, leaving anyone trying to speak with her exposed to the Stygian winds. The Frozen Princess could be seen across a perilous 60 ft (18 m) gulf with flying devils spiralling overhead, accompanied by an ancient white dragon.
https://forgottenrealms.fandom.com/wiki/Levistus#Realm
Alternative Name(s)
The Fifth Layer
Type
Dimensional plane
Location under
Owner/Ruler
Ruling/Owning Rank
Owning Organization
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