My patron goddess, last and lowest form
Of Hecate through whom the shades and I
Hold silent converse; warder of the gate
Who castest human offal to the dog:
Ye sisters who shall spin the threads again;
Pharsalia (aka "The Civil War") BOOK VI
A shade (shadow) is an archaic synonym for ghost. In ancient times it was believed that a person's shadow was the soul. At death the shadow/soul would seperate from the body and cross over into the underworld or afterlife. This belief explains why certain demonic creatures like vampires do not cast shadows, as they possess no souls.
Spirits in the form of moving shadows are a staple in ghost lore.