Technically speaking, the River Styx isn't an underworld realm – it is a liminal space that touches on all known underworld realms. Manifesting as a great flowing river whose far shore is dimly visible through the mist, the River Styx is cast eternally in twilight. Its near shore is thick clay-like mud a deep black color, with patches of stones and the occasional bone interrupting its smoothness. The shore descends down to the water's edge where a panoply of tall grasses occasionally ripple and whisper in a wind that living flesh never feels.
The calm surface of the black waters are occasionally broken by the shade of one of the dead, surfacing to try and clutch at a life it has already lost. These spirits are often borne away by a tide that affects only them.
Because the Styx touches on all places in the earthly realm, it is possible for those canny in its metaphysical geography to enter the realm, travel a carefully calculated distance, and then re-emerge in the earthly realm a significant distance away from where they began. There is a danger navigating the shoreline in this way, however: those who wander these realms over-long may find the call of the river pulling at their body and spirit in dangerous ways.