River Lethe
The River Lethe is noted as flowing through Nessus, Baator, and Hades‘ Realm, Pluton, Gray Waste. In fact, the river is something of a mystery. It rises somewhere in Chronias, Mt. Celestia, alongside the Euonë, it is told. It then flows down every layer of the Mountain, running in a broad, straight canal through Yetsirah, and thence wends its way down to the slopes of Lunia. However, it does not flow into the ocean there, but rather enters a cave and flows away into the depths of the Mountain, emerging in Nessus, of all places.
Quite how (or indeed why) Lunia is linked to Nessus is unclear, because the Nessus end of the river is always heavily guarded by baatezu. Some say that the enigmatic Lord of the Ninth seals it himself from time to time. Having thus entered Nessus, the Lethe flows parallel to the Styx into Pluton, where it eventually joins it in Hades’ Realm. Lethe is called the River of Forgetfulness: Whoever drinks it forgets everything, just as with the Styx. However, unlike the Styx there is a way of retrieving lost memories from Lethe: simply walk upstream one mile for every year of life forgotten.
Source: Alex Roberts