The Underworld River Restricted Area was one of a few restricted areas in the Wildlands.... The reason such areas came to be called 'restricted' was that ordinary soul cultivators would have a difficult time even getting into them, and if they succeeded in that effort, would most likely die as a result. The only way to safely traverse such areas was with special command medallions.
Not surprisingly, the Underworld River Restricted Area actually contained a river. During the day, it looked like nothing more than an ordinary, dried up riverbed. However, after midnight passed... the Underworld River would manifest, flowing through the area for some distance before disappearing off into the night.
Thus, the area came to be a restricted area. After all, the Underworld River was normally illusory, and would only appear in the world of men under special circumstances or using special techniques.
