The Pool of Loss is a portal to Hades. The drifting shapes are soul spectres from that plane.
These harmless phantoms are waiting to enter the Realm of the Dead. They can see, move freely and speak, but have no tangible existence, (appearing only as translucent images), and can neither attack nor be attacked on the Prime Material Plane. They can only have come here through the pool, which is customarily an open gate between the Realms and the Land of Always Night.
In ancient days, barbaric northern peoples hurled the bodies of great leaders into the pool, so that their greatness would not be lost to the service of the clan. The pool today exists as a well in this hidden cavern beneath Mount Waterdeep.