The shadow, in fact, occupies an important part of superstition in relation to
the body because its presence or absence was originally connected to very deep
religions and pagan beliefs. Remember Peter Pan's lack of shadow?
The most ancient understanding of the body and soul was that the soul was
attached to the body in such a way that under certain circumstances it could be
removed and would thereafter drift away as the soul on its way to afterlife.
If a vampire came up behind you and nailed your shadow to the wall, it took
possession of your body. If some poor unfortunate died in the night and
the shadow/soul drifted away, there was danger that it would cross water and be
unable to reach the afterlife. Instead it would go back into the dead body
and become the 'undead' or the 'walking dead', a form of a vampire. This
gave rise to the habit of putting lids on rain-water barrels and added
significance to bridges.
LORE & SOURCES