Sending: Speak With Dead, but better?

[Millstone85]

a creature's memories reside with the animating spirit.

Any idea how this connects/coexists with the River Styx?

Before arriving in Baator, lawful evil souls take a dive in the River Styx, which steals their memories and turns them into silt. The souls then emerge on the river's banks as near-mindless lemures.

A similar fate awaits souls in Hades (as larvas) and in the Abyss (as manes), which I would guess also involves the River Styx.

Anyway, that means these souls did have memories before the fiendish waters stole them.