River Styx

The River Styx runs through the entirety of the Abyss. Merely tasting or touching its waters causes most creatures to become stupefied, and drinking fully or being immersed for too long can render that condition permanent, robbing a creature of all its memories. Fiends don't fear being momentarily exposed to the Styx, but--with the notable exception of Hydroloths, Merrenoloths, and Amnizus--even they can't retain their memories if they drink from the River Styx or swim for too long in its waters.
Level 1 exposure causes memory loss. The exposed creature must succeed on a DC 18 Intelligence saving throw or be stunned for one hour and forget everything it sensed, experienced, and learned during the last 1d10 days. If it's been less than that amount of time since they last lost memories to the River Styx, they lose older memories (eg a creature who lost 4 days of memories a day ago and then fails their save and rolls a 7 will lose the day since their last exposure and 6 days before the start of their previous lost time, for a total of 11 lost days).
Level 2 exposure causes memory loss similar to level 1 exposure, except there is no saving throw against it and it takes 3d10 days instead of 1d10. In addition, the exposed creature must make a DC 15 Intelligence saving throw or suffer effects similar to the gentle feeblemind spell.
For a non-fiend, spending at least an hour within proximity to the waters (breathing the vapors that evaporate off of it) constitutes level 1 exposure, while this has no effect on a fiend. Touching the water for one hour or drinking any of the water from it results in level 1 exposure for fiends and level 2 exposure for non-fiends. In addition, for any creature, losing their saving throw against level 1 exposure repeatedly can result in level 2 exposure - if the creature fails 1d10 + 5 saves within a week, all further exposure counts as level 2 exposure.
The Pre-Sundering Fiends mostly avoided the river, and the only ones who regularly traveled it were the Hydroloth and Merrenoloth, who are inured to its effects and capable of living among the waters in relative safety. When Vutualli/Rhother/Bogtin, his Devils, and the revenge-obsessed Demons came to the Abyss after the Sundering, they soon learnt of the dangers of the river.
However, they soon realized that despite the dangers, the ever-shifting waters of the river are invaluable for anyone wishing to travel quickly from place to place within the Abyss. The River Styx frustrates every attempt to map it or predict its course. Although anyone can try to navigate it, only merrenoloths can faultlessly negotiate the Styx's treacherous waters. For a price, merrenoloths began to ferry fiendish passengers safely and swiftly across The Abyss. The greater the distance and the more perilous the passengers to be ferried, the higher the price.
When the Curse kicked off the soul trade in the Abyss, Demons and Devils found another use for the river, forcing the spirits of the dead to consume and bathe in Styx water to decompose them into more useable forms. Amnizu were among the forerunners of this process.

Type

River

Location under

Abyss