Not so with Men. Men's souls are MADE to leave Middle Earth, and so are more weakly attached to it. Imagine fell servents of the BW(s) that consist of Fell, corrupted, Unbodied Elven spirits possessing the bodies of Men (much like the Wights were Unbodied spirits inhabiting the dead bodies of Dunedain Kings, and the Wargs were Unbodied spirits inhabiting the forms of Wolves). These could be your 'Shadow Elves', essentially Fell, Undead servants of the BW(s). You could either rule that they look just like the Men they posses, or that they corrupt their form, taking on an obviously evil and menacing appearance, maybe a half-wight/wraith, not wholly trapped in the Spirit World (as the Ringwraiths were), but in both worlds at once because they actually possess living bodies (with some help from the BW(s)). In any case, these 'Shadow' servents would almost certainly radiate Fellness and Fear.