In Greek mythology, Lethe (Λήθη; "forgetfulness, oblivion") was both one of the five rivers of the underworld and the personification of forgetfulness. All who drank from the river of Lethe experienced complete forgetfulness, and the shades of the dead were said to drink from it so that their memories would be washed away before reincarnation. As a personified figure, Lethe was numbered among the children of Eris (Strife). Her opposite was Mnemosyne, the personification of memory.
Adapted from Wikipedia, "Lethe" · CC BY-SA 4.0
I.Family
The river Lethe was often associated with Lethe, the personification of forgetfulness and oblivion, who was reckoned a daughter of Eris (Strife). As such she belonged to the brood of grim abstractions descended from Strife, and stood as a counterpart to Mnemosyne, the personification of memory.
II.River of the Underworld
Lethe was one of the five rivers of Hades's underworld, and all those who drank from it experienced complete forgetfulness. According to mythology, the shades of the dead were only able to be reincarnated after they drank from the Lethe, which would wash away all their memories.
The river was situated next to Hades's palace in the underworld, beneath a cypress tree. Ovid wrote that the river flowed through the cave of Hypnos, god of sleep, where its murmuring would induce drowsiness.
III.Cult and Mystery Religion
Mystery religions taught an alternative to the oblivion of Lethe: those who instead drank from the spring of Mnemosyne would remember everything and attain omniscience.
At the oracle of Trophonius in Boeotia, there were rivers of Lethe and Mnemosyne from which worshippers would drink before making oracular consultations, ritually shedding their memories before receiving the prophecy.