WordNet sense 1 (a spy employed to follow someone and report their movements):

tail, shadower

WordNet sense 2 (an unilluminated area):

dark, darkness

WordNet sense 3 (an indication that something has been present):

tincture, trace, vestige

WordNet sense 4 (something existing in perception only):

apparition, phantasm, phantom, phantasma, fantasm

WordNet sense 5 (cast a shadow over):

shade, shade off

WordNet sense 6 (make appear small by comparison):

dwarf, overshadow